Localized superconcentrated electrolytes for stable cycling of electrochemical devices

ABSTRACT

Embodiments of localized superconcentrated electrolytes (LSEs) for stable operation of electrochemical devices, such as rechargeable batteries, supercapacitors, and sensors, are disclosed. Electrochemical devices, such as rechargeable batteries, supercapacitors, and sensors, including the LSEs are also disclosed. The LSEs include an active salt, a solvent in which the active salt is soluble, and a diluent in which the active salt is insoluble or poorly soluble. In certain embodiments, such as when the solvent and diluent are immiscible, the LSE further includes a bridge solvent.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier filing date of U.S. Provisional Application No. 62/466,267, filed Mar. 2, 2017, which is incorporated by reference in its entirety herein.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

FIELD

This invention is directed to localized superconcentrated electrolytes for stable cycling of electrochemical devices, the electrolytes including an active salt, a solvent in which the active salt is soluble, and a diluent in which the active salt is insoluble or poorly soluble.

BACKGROUND

Lithium metal is considered the ultimate anode material for achieving high energy density of batteries because it has the highest specific capacity (3860 mAh g⁻¹, 2061 mAh cm⁻³), and the lowest electrochemical potential (−3.04 V vs. standard hydrogen electrode). However, critical challenges including lithium dendrite growth and limited coulombic efficiency (CE) during cycling have inhibited the practical use of lithium batteries in rechargeable batteries (Xu et al., Energy Environ. Sci. 2014, 7:513-537). Recently, research efforts have revived to improve the stability of Li metal anodes, aiming to realize the practical applications of Li metal batteries. Representative approaches include the use of electrolyte additives, superconcentrated electrolytes, polymeric or solid-state electrolytes, and selective/guided Li deposition.

Recently, superconcentrated electrolytes have received intensive research attention for Li metal batteries owing to their various unique functionalities. One particular study by Qian et al. reported that the use of superconcentrated electrolytes composed of single bis(fluorosulfonyl)imide (LiFSI) salt in dimethoxyethane (DME) could enable the high-rate cycling of a Li metal anode with high CEs (up to 99.1%) without dendrite formation (Qian et al., Nat. Commun. 2015, 6:6362). However, superconcentrated electrolytes usually suffer from high viscosity, and/or poor wetting of the separator and the cathode. It is not practically feasible to inject a highly viscous electrolyte into large-format lithium ion batteries, such as prismatic or cylindrically shaped batteries, during the manufacturing process. Furthermore, the high cost of the high concentration lithium salt also hinders the practical use of superconcentrated electrolytes.

SUMMARY

Embodiments of localized superconcentrated electrolytes (LSEs) and electrochemical systems including LSEs are disclosed. A LSE includes an active salt, a solvent in which the active salt is soluble, and a diluent, wherein the active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the solvent. In some embodiments, the solvent comprises dimethoxyethane (DME), dimethyl carbonate (DMC), 1,3-dioxolane (DOL), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl sulfoxide (DMSO), ethyl vinyl sulfone (EVS), tetramethylene sulfone (TMS), ethyl methyl sulfone (EMS), ethylene carbonate (EC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-vinyl-1,3-dioxolan-2-one (VEC), dimethyl sulfone, ethyl methyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, methylene ethylene carbonate (MEC), 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, water, or any combination thereof.

In any or all of the above embodiments, (i) the active salt may have a molar concentration in the electrolyte within a range of from 0.5 M to 2 M; (ii) the active salt may have a molar concentration in the solvent of greater than 3 moles of active salt per liter of the solvent; or (iii) both (i) and (ii). In some embodiments, the molar concentration of the active salt in the electrolyte is at least 20% less than a molar concentration of the active salt in the solvent in the absence of the diluent.

In any or all of the above embodiments, (i) a molar ratio of the active salt to the solvent may be within a range of from 0.33 to 1.5; (ii) a molar ratio of the solvent to the diluent may be within a range of from 0.2 to 5; or (iii) both (i) and (ii). In any or all of the above embodiments, at least 90% of molecules of the solvent may be associated with cations of the active salt. In any or all of the above embodiments, fewer than 10% of molecules of the diluent may be associated with cations of the active salt.

In any or all of the above embodiments, the active salt may include a lithium salt or lithium salts mixture, a sodium salt or sodium salts mixture, or a magnesium salt or magnesium salts mixture. In some embodiments, the active salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), lithium bis(oxalato)borate (LiBOB), sodium bis(oxalato)borate (NaBOB), LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, lithium difluoro oxalato borate anion (LiDFOB), LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof. In certain of the foregoing embodiments, the active salt is (i) LiFSI, LiTFSI, or a combination thereof, or (ii) NaFSI, NaTFSI, or a combination thereof; the solvent is DMC, DME, DOL, EMC, or a combination thereof; and the active salt has a molar concentration in the electrolyte within a range of from 0.75 M to 1.5 M.

In any or all of the above embodiments, the diluent may comprise a fluoroalkyl ether (also referred to as a hydrofluoroether (HFE)). In some embodiments, the diluent comprises 1,1,2,2-tetrafluoroethyl-2,2,2,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), or any combination thereof. In any or all of the above embodiments, the solvent and the diluent may be miscible.

In some embodiments, a LSE includes an active salt; a solvent comprising a carbonate solvent, an ether solvent, a phosphate solvent, dimethyl sulfoxide, water, or a combination thereof, wherein the active salt is soluble in the solvent; a diluent, wherein the diluent is immiscible with the solvent and wherein active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the solvent; and a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent. Exemplary bridge solvents include acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), or any combination thereof.

In certain embodiments where the solvent and diluent are immiscible, the solvent comprises water. In such embodiments, the active salt may be (i) LiFSI, LiTFSI, LiBOB, LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiCl₄, LiDFOB, LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof, or (ii) NaFSI, NaTFSI, NaBOB, or a combination thereof.

Some embodiments of a battery as disclosed herein include (i) an electrolyte comprising an active salt, a solvent comprising a carbonate solvent, an ether solvent, a phosphate solvent, dimethyl sulfoxide, water, or a combination thereof, wherein the active salt is soluble in the solvent, and a diluent, wherein the active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the solvent, and wherein the active salt has a concentration in the electrolyte within a range of 0.75 to 2 M; (ii) an anode; and (iii) a cathode, wherein the battery has a coulombic efficiency ≥95%. Exemplary solvents include DME, DOL, DMC, EMC, DEC, DMSO, EVS, TMS, EMS, EC, VC, FEC, VEC, dimethyl sulfone, ethyl methyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, MEC, 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, water, or any combination thereof.

In one embodiment of the battery, (i) the anode is lithium metal; (ii) the active salt comprises LiFSI, LiTFSI, LiBOB, LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, LiDFOB, LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof; (iii) the solvent comprises DMC, DME, DOL, EVS, TMS, EMS, EC, VC, FEC, VEC, dimethyl sulfone, ethyl methyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, MEC, 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, or any combination thereof; (iv) the diluent comprises TTE, BTFE, TFTFE, MOFB, EOFB, or any combination thereof; and (v) the cathode is Li_(1+w)Ni_(x)Mn_(y)Co_(z)O₂ (x+y+z+w=1, 0≤w≤0.25), LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1), LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Mn_(1.5)O₄ spinel, LiMn₂O₄, LiFePO₄, Li_(4−x)M_(x)Ti₅O₁₂ (M=Mg, Al, Ba, Sr, or Ta; 0≤x≤1), MnO₂, V₂O₅, V₆O₁₃, LiV₃O₈, LiM^(C1) _(x)M^(C2) _(1−x)PO₄ (M^(C1) or M^(C2)=Fe, Mn, Ni, Co, Cr, or Ti; 0≤x≤1), Li₃V_(2−x)M¹ _(x)(PO₄)₃ (M¹=Cr, Co, Fe, Mg, Y, Ti, Nb, or Ce; 0≤x≤1), LiVPO₄F, LiM^(C1) _(x)M^(C2) _(1−x)O₂ ((M^(C1) and M^(C2) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1), LiM^(C1) _(x)M^(C2) _(y)M^(C3) _(1−x−y)O₂ ((M^(C1), M^(C2), and M^(C3) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1; 0≤y≤1), LiMn_(2−y)X_(y)O₄ (X=Cr, Al, or Fe, 0≤y≤1), LiN_(0.5−y)X_(y)Mn_(1.5)O₄ (X=Fe, Cr, Zn, Al, Mg, Ga, V, or Cu; 0≤y<0.5), xLi₂MnO₃.(1−x)LiM^(C1) _(y)M^(C2) _(z)M^(C3) _(1−y−z)O₂ (M^(C1), M^(C2), and M^(C3) independently are Mn, Ni, Co, Cr, Fe, or mixture thereof; x=0.3-0.5; y≤0.5; z≤0.5), Li₂M²SiO₄ (M²=Mn, Fe, or Co), Li₂M²SO₄ (M²=Mn, Fe, or Co), LiM²SO₄F (M²=Fe, Mn, or Co), Li_(2−x)(Fe_(1−y)Mn_(y))P₂O₇ (0≤y≤1), Cr₃O₈, Cr₂O₅, a carbon/sulfur composite, or an air electrode.

In an independent embodiment of the battery, (i) the anode is sodium metal; (ii) the active salt comprises NaFSI, NaTFSI, or a combination thereof; (iii) the solvent comprises DMC, DME, DOL, or any combination thereof; (iv) the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; and (v) the cathode is NaFePO₄, Na₂FePO₄F, Na₂FeP₂O₇, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, NaVPO₄F, NaVPOPOF, Na_(1.5)VOPO₄F_(0.5), NaCo₂O₄, NaFeO₂, Na_(x)MO₂ where 0.4<x≤1, and M is a transition metal or a mixture of transition metals, Na_(2/3)Ni_(1/3)Mn_(2/3)O₂, Na_(2/3)Fe_(1/2)Mn_(1/2)O₂, Na_(2/3)Ni_(1/6)Co_(1/6)Mn_(2/3)O₂, NaNi_(1/3)Fe_(1/3)Mn_(1/3)O₂, NaNi_(1/3)Fe_(1/3)Co_(1/3)O₂, NaNi_(1/2)Mn_(1/2)O₂, a Prussian white analogue cathode, or a Prussian blue analogue cathode.

In some embodiments of the battery, the solvent and the diluent are immiscible, and the electrolyte further includes a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent. In one embodiment where the solvent and the diluent are immiscible, (i) the active salt comprises LiFSI, LiTFSI, or a combination thereof; (ii) the solvent is H₂O; (iii) the bridge solvent comprises AN, DMC, DEC, PC, DMSO, DOL, DME, diglyme, triglyme, tetraglyme, or any combination thereof; (iv) the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; (v) the anode is Mo₆S₈, TiO₂, V₂O₅, Li₄Mn₅O₁₂, Li₄Ti₅O₁₂, a C/S composite, or a polyacrylonitrile-sulfur composite; and (vi) the cathode is LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1), LiFePO₄, LiMnPO₄, LiFe_(x)Mn_(1−x)PO₄, LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Mn_(1.5)O₄ spinel, or LiMn₂O₄. In an independent embodiment where the solvent and the diluent are immiscible, (i) the active salt comprises NaFSI, NaTFSI, or a combination thereof; (ii) the solvent is H₂O; (iii) the bridge solvent comprises AN, DMC, DEC, PC, DMSO, DOL, DME, diglyme, triglyme, tetraglyme, or any combination thereof; (iv) the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; (v) the anode is NaTi₂(PO₄)₃; TiS₂, CuS, FeS₂, NiCo₂O₄, Cu₂Se, or Li_(0.5)Na_(0.5)Ti₂(PO₄)₃; and (vi) the cathode is Na₄Co₃(PO₄)₂P₂O₇, Na₄Co_(2.4)Mn_(0.3)Ni_(0.3)(PO₄)₂P₂O₇, Na₇V₄(P₂O₇)₄PO₄, NaFePO₄, Na₂FePO₄F, Na₂FeP₂O₇, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, NaVPO₄F, NaVPOPOF, Na_(1.5)VOPO₄F_(0.5), Na₂Fe₂(SO₄)₃, NaFeO₂, and Na_(x)MO₂ where 0.4<x≤1, and M is a transition metal or a mixture of transition metals (e.g., NaCrO₂, NaCoO₂, Na_(x)CoO₂ (0.4≤x≤0.9), Na_(2/3)[Ni_(1/3)Mn_(1/2)Ti_(1/3)]O₂, Na_(2/3)Ni_(1/3)Mn_(2/3)O₂, Na_(2/3)Fe_(1/2)Mn_(1/2)O₂, Na_(2/3)Ni_(1/6)O_(1/6)Mn_(2/3)O₂, NaNi_(1/3)Fe_(1/3)Mn_(1/3)O₂, NaNi_(1/3)Fe_(1/3)Co_(1/3)O₂, NaNi_(1/2)Mn_(1/2)O₂, a Prussian white analogue cathode, or a Prussian blue analogue cathode.

The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a schematic illustration of a superconcentrated electrolyte (SE) comprising a lithium salt and a solvent.

FIG. 2 is a schematic illustration of an exemplary localized superconcentrated electrolyte (LSE) comprising a lithium salt, a solvent in which the lithium salt is soluble, and a diluent, i.e., a component in which the lithium salt is insoluble or poorly soluble compared to the solvent.

FIG. 3 is a schematic illustration of an exemplary “bridge” solvent molecule between a water molecule and a diluent molecule.

FIG. 4 is a schematic diagram of a battery.

FIGS. 5A and 5B show initial lithium deposition/stripping voltage profiles (FIG. 5A) and coulombic efficiency as a function of cycle number (FIG. 5B) for Li∥Cu cells having a lithium areal deposition capacity of 0.5 mAh cm⁻², tested at 1 mA cm⁻² with various concentrated electrolytes comprising lithium salts in carbonate solvents.

FIG. 6 is a graph of coulombic efficiency as a function of cycle number for Li∥Cu cells having a lithium areal deposition capacity of 1 mAh cm⁻², tested at 0.5 mA cm⁻² with electrolytes comprising concentrated LiFSI in ethyl methyl carbonate (EMC) with and without fluoroalkyl ether diluents.

FIG. 7 shows digital photographs demonstrating that addition of bis(2,2,2-trifluoroethyl) ether (BTFE) to an LiFSI/EMC electrolyte improved wetting of a battery separator.

FIGS. 8A and 8B are graphs showing the cycling stability of Li∥NMC761410 (LiNi_(0.76)Mn_(0.14)Co_(0.10)O₂) cells containing concentrated LiFSI/EMC electrolyte without and with BTFE diluent at C/3 (FIG. 8A) and 10 rates (FIG. 8B) (BTFE: bis(2,2,2-trifluoroethyl) ether).

FIGS. 9A and 9B how initial lithium deposition/stripping voltage profiles (FIG. 9A) and coulombic efficiency as a function of cycle number (FIG. 9B) for Cu∥Li cells having a lithium areal deposition capacity of 0.5 mAh/cm² with electrolytes comprising 7.5 mol/kg LiBF₄/PC and 2.5 mol/kg LiBF₄/PC-TTE (PC:TTE=2:1 v:v) (TTE: 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether).

FIGS. 10A-10D are Li plating/stripping profiles of Li∥Cu cells using conventional electrolyte (1.0 M LiPF₆/EC-EMC (4:6, w)) (FIG. 10A), 1.2 M LiFSI/DMC (FIG. 10B), 3.7 M LiFSI/DMC (FIG. 10C), and 5.5 M LiFSI/DMC (FIG. 10D).

FIGS. 11A-11D are Li plating/stripping profiles of Li∥Cu cells using concentrated 3.8M LiFSI/DMC-BTFE (1:0.5) (FIG. 11A), 2.5M LiFSI/DMC-BTFE (1:1) (FIG. 11B), 1.8M LiFSI/DMC-BTFE (1:1.5) (FIG. 11C), and 1.2M LiFSI/DMC-BTFE (1:2) (FIG. 11D). The ratios in the parentheses indicate the molar ratios of DMC:BTFE.

FIGS. 12A-12D are scanning electron microscopy images of lithium plated onto copper substrates after 100 cycles (1 mA/cm² to 0.5 mAh/cm²) from 1.0 M LiPF₆/EC-EMC (FIG. 12A), 5.5 M LiFSI/DMC (FIG. 12B), 3.7 M LiFSI/DMC (FIG. 12C), and 1.2 M LiFSI/DMC-BTFE (1:2) (FIG. 12D) electrolytes.

FIG. 13 is a graph of coulombic efficiency vs. cycle number for conventional electrolyte, dilute LiFSI/DMC electrolytes, superconcentrated LiFSI/DMC electrolyte, and LSE of 1.2 M LiFSI/DMC-BTFE (1:2).

FIG. 14 is a graph of conductivity vs. temperature for conventional electrolyte, dilute LiFSI/DMC electrolytes, superconcentrated LiFSI/DMC electrolyte, and certain LSEs as disclosed herein.

FIGS. 15A and 15B are graphs demonstrating the performance (voltage vs. capacity) of LiliLi symmetric cells in SE of 5.5 M LiFSI/DMC (FIG. 15A) and LSE of 1.2 M LiFSI/DMC-BTFE (1:2) (FIG. 15B) at varying current densities.

FIG. 16 is a graph of current vs. voltage illustrating the anodic stability of SE of 5.5 M LiFSI/DMC and certain LSEs as disclosed herein.

FIGS. 17A-17D are SEM images showing lithium plated onto copper substrates from 1.2 M LiFSI/DMC (FIGS. 17A, 17B) and 3.7 M LiFSI/DMC (FIGS. 17C, 17D); FIGS. 17A and 17C are cross-sectional views; FIGS. 17B and 17D are top views.

FIGS. 18A-18D show the electrochemical behavior of Li∥NMC batteries with different electrolytes. FIG. 18A shows the cycling stability and coulombic efficiencies. FIGS. 18B-18D show typical voltage profiles in 1.0 M LiPF₆/EC-EMC (FIG. 18B), 5.5 M LiFSI/DMC (FIG. 18C), and 1.2 M LiFSI/DMC-BTFE (1:2) (FIG. 18D).

FIG. 19 shows the rate performance of Li∥NMC batteries using different electrolytes; the batteries were charged at a constant C/5 rate but discharged at an increasing C rate; 1C=2.0 mA/cm².

FIG. 20 shows the rate performance of Li∥NMC batteries using different electrolytes; the batteries were discharged at a constant C/5 rate but charged at an increasing C rate; 1C=2.0 mA/cm².

FIGS. 21A-21F are SEM images showing morphology of Li metal after plating on Cu substrates in different electrolytes. FIGS. 21A, 21C, and 21E are cross-section views; FIGS. 21B, 21D, and 21F are top views of Li metal after plating on Cu substrates. The electrolytes were 1.0 M LiPF₆/EC-EMC (FIGS. 21A, 21B), 5.5 M LiFSI/DMC (FIGS. 21C, 21D), and 1.2 M LiFSI/DMC-BTFE (1:2) (FIGS. 21E, 21F).

FIGS. 22A-22C are SEM images showing morphology of Li metal after plating on Cu substrates in 1.2 M LiFSI/DMC-BTFE (1:2) at current densities of 2 mA/cm² (FIG. 22A), 5 mA/cm² (FIG. 22B), and 10 mA/cm² (FIG. 22C).

FIG. 23 shows the coulombic efficiency (CE) of Li∥Cu cells using concentrated LiFSI/DME electrolyte and those with TTE or BTFE diluent as a function of cycle number tested at 0.5 mA cm⁻² with Li deposition areal capacity of 1 mAh cm⁻².

FIG. 24 shows the cycling stability of Li∥LiFePO₄ (LFP) cells containing concentrated 4 M LiFSI/DME electrolyte without and with TTE or BTFE diluent at 10 rate after 3 formation cycles at C/10, in the voltage range of 2.5˜3.7 V.

FIGS. 25A and 25B show initial Na deposition/stripping voltage profiles (FIG. 25A) and CE of Na∥Cu cells as a function of cycle number tested at 0.65 mA cm⁻² after 2 formation cycles at 0.26 mA cm⁻², with Na deposition areal capacity of 1.3 mAh cm⁻² (FIG. 25B).

FIGS. 26A and 26B show initial charge/discharge voltage profiles (FIG. 26A) and cycling stability (FIG. 26B) of Na∥Na₃V₂(PO₄)₃ cells containing superconcentrated NaFSI/DME electrolyte and LSEs with TTE diluent at C/3.

FIGS. 27A and 27B show the charge and discharge capacities of Na∥Na₃V₂(PO₄)₃ cells containing 5.2 M NaFSI/DME (FIG. 27A) and 2.3 M NaFSI/DME-TTE (DME:TTE molar ratio 1:1) (FIG. 27B) electrolytes.

FIGS. 28A and 28B show initial Na deposition/stripping voltage profiles (FIG. 28A) and CE of Na∥Cu cells as a function of cycle number tested at 1 mA cm⁻² after 2 formation cycles at 0.2 mA cm⁻², (FIG. 28B) with 5.2 M NaFSI/DME, 3.1 M NaFSI/DME-BTFE (1:1), 2.1 M NaFSI/DME-BTFE (1:2), and 1.5 M NaFSI/DME-BTFE (1:3) electrolytes. The ratios in the parentheses indicate the molar ratios of DME:BTFE in different BTFE diluted LSEs.

FIGS. 29A-29C show the electrochemical performance of Na∥Na₃V₂(PO₄)₃ cells using 5.2 M NaFSI/DME and BTFE-diluted NaFSI/DME-BTFE electrolytes. FIG. 29A shows the initial Na plating/stripping profiles; FIG. 29B shows the cycling stability over 100 cycles; FIG. 29C shows the charge and discharge capacities of NaFSI/DME-BTFE (1:1:2 in mol) over 100 cycles.

FIGS. 30A and 30B show initial Li deposition/stripping voltage profiles (FIG. 30A) and CE (FIG. 30B) of the Li∥Cu cells as a function of cycle number tested at 1 mA cm⁻² after 2 formation cycles at 0.2 mA cm⁻² with Li deposition areal capacity of 1 mAh cm⁻² using low concentration 1M LiTFSI/DOL-DME, concentrated 3.3M LiTFSI/DOL-DME electrolyte, LSE of 1.06 M LiTFSI/DOL-DME-TTE electrolyte.

FIGS. 31A-31C show the electrochemical performance of Li—S cells containing low concentration 1M LiTFSI/DOL-DME, concentrated 3.3M LiTFSI/DOL-DME electrolyte and LSE of 1.06 M LiTFSI/DOL-DME-TTE electrolyte; FIG. 31A is the initial charge/discharge voltage profiles, FIG. 31B is the cycling performance, and FIG. 31C shows the CE of the Li—S cells as a function of cycle number evaluated at 0.1 C (168 mA g⁻¹).

FIG. 32 shows charge/discharge profiles of Li—O₂ cells using LiTFSI-3DMSO (dimethyl sulfoxide) (2.76 M) and LiTFSI-3DMSO-3TTE (1.23 M) electrolytes with limited discharge capacity of 600 mAh g⁻¹ at a current density of 0.1 mA cm⁻².

FIG. 33 shows cyclic voltammograms of concentrated aqueous electrolyte before and after dilution with TTE with the assistance of different ‘bridge’ solvents (acetonitrile (AN), dimethyl carbonate (DMC), propylene carbonate (PC), and DMSO), using a stainless steel working electrode and counter electrode, and Ag/AgCl as reference electrode at a scan rate of 10 mV s⁻¹. The potential was converted to those versus to Li/Li⁺ redox couple.

FIGS. 34A and 34B, respectively, show first cycle and second cycle cyclic voltammograms of concentrated aqueous electrolyte diluted with different amounts of TTE with the assistance of PC. Stainless steel was the working electrode and counter electrode, and Ag/AgCl was the reference electrode. Scan rate of 10 mV s⁻¹. The potential was converted to those versus to Li/Li⁺ redox couple.

FIG. 35 shows optimized molecular structures of DMC and BTFE solvent molecules, LiFSI salt, and DMC+LiFSI and BTFE+LiFSI solvent-salt pairs. The Li, O, C, H, S, N, and F atoms are colored as magenta, red, gray, white, yellow, blue, and light blue, respectively.

FIGS. 36A-36F are molecular models showing adsorption of solvent molecules DMC (FIG. 36A) and BTFE (FIG. 36B), LiFSI salt (FIG. 36C), and DMC-LiFSI solvent-salt pairs (FIGS. 36D-36F) on the lithium (100) anode surface. The upper and lower images in each pair are the top and side view structures, respectively.

FIGS. 37A-37C are molecular models of electrolyte/salt mixtures from AIMD simulations at 303 K-LiFSI-DMC (1:1.1) (FIG. 37A); LiFSI-DMC-BTFE (0.94:1.1:0.55) (FIG. 37B); LiFSI-DMC-BTFE (0.51:1.1:2.2) (FIG. 37C); the ratios in the parentheses indicate the molar ratios of LiFSI:DMC:BTFE.

FIG. 38 is a graph of the radial distribution functions of Li—O_(DMC) and Li—O_(BTFE) pairs calculated from AIMD simulation trajectories at 303 K.

FIGS. 39A and 39B are Raman spectra of pure DMC solvent, pure BTFE solvent, and solvent mixture of DMC-BTFE (2:1); FIG. 39B is an enlarged view of FIG. 39A in the wavenumber range of 2000-200 cm⁻¹.

FIGS. 40A and 40B are Raman spectra of different concentrations of LiFSI/DMC solutions (FIG. 40A) and different concentrations of BTFE diluted LiFSI/DMC-BTFE solutions (FIG. 40B).

FIG. 41 shows diffusion coefficients (Ds) of Li⁺, FSI⁻ and solvent molecules (DMC and BTFE) at 30° C. across the samples plotted with the inverse of viscosity (η⁻¹), which is denoted with stars. The bars, from left to right, indicate the following species where present—BTFE, DMC, Li, FSI.

FIG. 42 shows diffusion ratios of BTFE, Li, and FSI in DMC—D_(BTFE)/D_(DMC), D_(Li)/D_(DMC) and D_(FSI)/D_(DMC) at 30° C.

FIGS. 43A-43C are graphs showing projected density of states (PDOS) for dilute electrolyte (LiFSI/DMC, LiFSI:DMC molar ratio 1:2) (FIG. 43A), superconcentrated electrolyte (5.5M LiFSI/DMC, LiFSI:DMC molar ratio 1:1) (FIG. 43B), and BTFE-diluted electrolyte (LiFSI/DMC-BTFE, LiFSI:DMC:BTFE molar ratio 1:2:4) (FIG. 43C) on the lithium anode surface.

DETAILED DESCRIPTION

Superconcentrated electrolytes, e.g. concentrated LiFSI/DME or concentrated LiFSI/DMC, can enable high coulombic efficiency (CE) operation of lithium metal anodes, due to the reduced presence of free solvent molecules compared to more dilute electrolytes and the formation of stabilized SEI layer. As used herein, the term “superconcentrated” means an active salt concentration of at least 3 M. However, these superconcentrated electrolytes suffer from high cost, high viscosity, and/or poor wetting toward the separator and the thick cathode electrode, hindering their practical use. Embodiments of localized superconcentrated electrolytes (LSE) are disclosed. Certain embodiments of the disclosed LSEs are stable in electrochemical cells with alkali metal, alkaline earth metal, or carbon-based (e.g., graphite) anodes and various cathode materials. The LSEs comprise an active salt, a solvent in which the active salt is soluble, and a diluent in which the active salt is insoluble or poorly soluble. In certain embodiments, the concentration of active salt is lowered by addition of the diluent without significant reduction in the performance of batteries using the LSE.

I. DEFINITIONS AND ABBREVIATIONS

The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.

Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, molarities, voltages, capacities, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.

Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order unless stated otherwise.

Definitions of common terms in chemistry may be found in Richard J. Lewis, Sr. (ed.), Hawley's Condensed Chemical Dictionary, published by John Wiley & Sons, Inc., 1997 (ISBN 0-471-29205-2).

In order to facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:

Active salt: As used herein, the term “active salt” refers to a salt that significantly participates in electrochemical processes of electrochemical devices. In the case of batteries, it refers to charge and discharge processes contributing to the energy conversions that ultimately enable the battery to deliver/store energy. As used herein, the term “active salt” refers to a salt that constitutes at least 5% of the redox active materials participating in redox reactions during battery cycling after initial charging.

AN: acetonitrile

Anode: An electrode through which electric charge flows into a polarized electrical device. From an electrochemical point of view, negatively-charged anions move toward the anode and/or positively-charged cations move away from it to balance the electrons leaving via external circuitry. In a discharging battery or galvanic cell, the anode is the negative terminal where electrons flow out. If the anode is composed of a metal, electrons that it gives up to the external circuit are accompanied by metal cations moving away from the electrode and into the electrolyte. When the battery is recharged, the anode becomes the positive terminal where electrons flow in and metal cations are reduced.

Associated: As used here, the term “associated” means coordinated to or solvated by. For example, a cation that is associated with a solvent molecule is coordinated to or solvated by the solvent molecule. Solvation is the attraction of solvent molecules with molecules or ions of a solute. The association may be due to electronic interactions (e.g., ion-dipole interactions and/or van der Waals forces) between the cation and the solvent molecule. Coordination refers to formation of one or more coordination bonds between a cation and electron lone-pairs of solvent atoms. Coordination bonds also may form between the cation and anion of the solute.

Bridge solvent: A solvent having amphiphilic molecules with a polar end or moiety and a nonpolar end or moiety.

BTFE: bis(2,2,2-trifluoroethyl) ether

Capacity: The capacity of a battery is the amount of electrical charge a battery can deliver. The capacity is typically expressed in units of mAh, or Ah, and indicates the maximum constant current a battery can produce over a period of one hour. For example, a battery with a capacity of 100 mAh can deliver a current of 100 mA for one hour or a current of 5 mA for 20 hours. Areal capacity or specific areal capacity is the capacity per unit area of the electrode (or active material) surface, and is typically expressed in united of mAh cm⁻².

Cathode: An electrode through which electric charge flows out of a polarized electrical device. From an electrochemical point of view, positively charged cations invariably move toward the cathode and/or negatively charged anions move away from it to balance the electrons arriving from external circuitry. In a discharging battery or galvanic cell, the cathode is the positive terminal, toward the direction of conventional current. This outward charge is carried internally by positive ions moving from the electrolyte to the positive cathode, where they may be reduced. When the battery is recharged, the cathode becomes the negative terminal where electrons flow out and metal atoms (or cations) are oxidized.

Cell: As used herein, a cell refers to an electrochemical device used for generating a voltage or current from a chemical reaction, or the reverse in which a chemical reaction is induced by a current. Examples include voltaic cells, electrolytic cells, and fuel cells, among others. A battery includes one or more cells. The terms “cell” and “battery” are used interchangeably when referring to a battery containing only one cell.

Coin cell: A small, typically circular-shaped battery. Coin cells are characterized by their diameter and thickness.

Conversion compound: A compound comprising one or more cations, which are displaced by another metal when a battery is discharged. For example, when iron (II) selenide (FeSe) is used as a cathode material, Fe is replaced by Na during discharge of a Na battery: 2Na⁺+2e ⁻+FeSe↔Na₂Se+Fe

Coulombic efficiency (CE): The efficiency with which charges are transferred in a system facilitating an electrochemical reaction. CE may be defined as the amount of charge exiting the battery during the discharge cycle divided by the amount of charge entering the battery during the charging cycle. CE of Li∥Cu or Na∥Cu cells may be defined as the amount of charge flowing out of the battery during stripping process divided by the amount of charge entering the battery during plating process.

DEC: diethyl carbonate

DMC: dimethyl carbonate

DME: dimethoxyethane

DMSO: dimethyl sulfoxide

DOL: 1,3-dioxolane

Donor number: A quantitative measure of Lewis basicity, such as a solvent's ability to solvate cations. A donor number is defined as the negative enthalpy value for the 1:1 adduct formation between a Lewis base and SbCl₅ in dilute solution in 1,2-dichloroethane, which has a donor number of zero. The donor number is typically reported in units of kcal/mol. Acetonitrile, for example, has a donor number of 14.1 kcal/mol. As another example, dimethyl sulfoxide has a donor number of 29.8 kcal/mol.

EC: ethylene carbonate

Electrolyte: A substance containing free ions that behaves as an electrically conductive medium. Electrolytes generally comprise ions in a solution, but molten electrolytes and solid electrolytes also are known.

EMC: ethyl methyl carbonate

EMS: ethyl methyl sulfone

EOFB: ethoxynonafluorobutane

EVS: ethyl vinyl sulfone

FEC: fluoroethylene carbonate

Immiscible: This term describes two substances of the same state of matter that cannot be uniformly mixed or blended. Oil and water are a common example of two immiscible liquids.

Intercalation: A term referring to the insertion of a material (e.g., an ion or molecule) into the microstructure of another material. For example, lithium ions can insert, or intercalate, into graphite (C) to form lithiated graphite (LiC₆).

LiFSI: lithium bis(fluorosulfonyl)imide

LiTFSI: lithium bis(trifluoromethylsulfonyl)imide

LiBOB: lithium bis(oxalato)borate

LiDFOB: lithium difluoro oxalato borate anion

LSE: localized superconcentrated electrolyte

MEC: methylene ethylene carbonate

MOFB: methoxynonafluorobutane

NaFSI: sodium bis(fluorosulfonyl)imide

NaTFSI: sodium bis(trifluoromethylsulfonyl)imide

NaBOB: sodium bis(oxalato)borate

PC: propylene carbonate

Separator: A battery separator is a porous sheet or film placed between the anode and cathode. It prevents physical contact between the anode and cathode while facilitating ionic transport.

Soluble: Capable of becoming molecularly or ionically dispersed in a solvent to form a homogeneous solution. As used herein, the term “soluble” means that an active salt has a solubility in a given solvent of at least 1 mol/L (M, molarity) or at least 1 mol/kg (m, molality).

Solution: A homogeneous mixture composed of two or more substances. A solute (minor component) is dissolved in a solvent (major component). A plurality of solutes and/or a plurality of solvents may be present in the solution.

Superconcentrated: As used herein, the term “superconcentrated electrolyte” refers to an electrolyte having a salt concentration of at least 3 M.

TFTFE: 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether

TMTS: tetramethylene sulfone or sulfolane

TTE: 1,1,2,2-tetrafluoroethyl-2,2,2,3-tetrafluoropropyl ether

VC: vinylene carbonate

VEC: 4-vinyl-1,3-dioxolan-2-one

II. LOCALIZED SUPERCONCENTRATED ELECTROLYTES

A superconcentrated electrolytes comprises a solvent and a salt with a salt concentration of at least 3 M. Some superconcentrated electrolytes have a salt concentration of at least 4 M or at least 5 M. In certain instances, the salt molality may be up to 20 m or more, e.g., aqueous LiTFSI. FIG. 1 is a schematic illustration of a conventional superconcentrated electrolyte comprising a solvent and a lithium salt. Desirably, all or a large majority of the solvent molecules are associated with a lithium cation in the superconcentrated electrolyte. A reduced presence of free, unassociated solvent molecules increases coulombic efficiency (CE) of a lithium metal anode, facilitates formation of a stabilized solid electrolyte interphase (SEI) layer, and/or increases cycling stability of a battery including the electrolyte. However, superconcentrated electrolytes have disadvantages, such as high material cost, high viscosity, and/or poor wetting of battery separators and/or cathodes. While dilution with additional solvent can resolve one or more of the disadvantages, dilution results in free solvent molecules and often decreases CE, hinders formation of the stabilized SEI layer, and/or decreases cycling stability of a battery.

Certain embodiments of the disclosed “localized superconcentrated electrolyte” (LSE) comprising a salt, a solvent in which the salt is soluble, and a diluent in which the salt is insoluble or poorly soluble can resolve some or all of the problems discussed above. FIG. 2 is a schematic illustration of an exemplary LSE including a lithium salt, a solvent in which the lithium salt is soluble, and a diluent in which the lithium salt is insoluble or poorly soluble. As shown in FIG. 2, the lithium ions remain associated with solvent molecules after addition of the diluent. The anions are also in proximity to, or associated with, the lithium ions. Thus, localized regions of solvent-cation-anion aggregates are formed. In contrast, the lithium ions and anions are not associated with the diluent molecules, which remain free in the solution. Evidence of this electrolyte structure with regions of locally concentrated salt/solvent and free diluent molecules is seen by Raman spectroscopy (see, e.g., Example 10, FIGS. 39A-B, 40A-B), NMR characterization, and molecular dynamics (MD) simulations. Thus, although the solution as a whole is less concentrated than the solution of FIG. 1, there are localized regions of high concentration where the lithium cations are associated with the solvent molecules. There are few to no free solvent molecules in the diluted electrolyte, thereby providing the benefits of a superconcentrated electrolyte without the associated disadvantages.

Embodiments of the disclosed localized superconcentrated electrolytes (LSEs) include an active salt, a solvent A, wherein the active salt is soluble in the solvent A, and a diluent, wherein the active salt is insoluble or poorly soluble in the diluent. As used herein, “poorly soluble” means that the active salt has a solubility in the diluent at least 10× less than a solubility of the active salt in the solvent A.

The solubility of the active salt in the solvent A (in the absence of diluent) may be greater than 3 M, such as at least 4 M or at least 5 M. In some embodiments, the solubility and/or concentration of the active salt in the solvent A is within a range of from 3 M to 10 M, such as from 3 M to 8 M, from 4 M to 8 M, or from 5 M to 8 M. In certain embodiments, such as when solvent A comprises water, the concentration may be expressed in terms of molality and the concentration of the active salt in the solvent A (in the absence of diluent) may be within a range of from 3 m to 25 m, such as from 5 m to 21 m, or 10 m to 21 m. In contrast, the molar or molal concentration of the active salt in the electrolyte as a whole (salt, solvent A, and diluent) may be at least 20% less than the molar or molal concentration of the active salt in the solvent A, such as at least 30% less, at least 40% less, at least 50% less, at least 60% less, or even at least 70% less than the molar or molal concentration of the active salt in the solvent A. For example, the molar or molal concentration of the active salt in the electrolyte may be 20-80% less, 20-70% less, 30-70% less, or 30-50% less than the molar or molal concentration of the active salt in the solvent A. In some embodiments, the molar concentration of the active salt in the electrolyte is within a range of 0.5 M to 3 M, 0.5 M to 2 M, 0.75 M to 2 M, or 0.75 M to 1.5 M.

The active salt is a salt, or combination of salts, that participates in the charge and discharge processes of a cell including the electrolyte. The active salt comprises a cation that is capable of forming redox pairs having different oxidation and reduction states, such as ionic species with differing oxidation states or a metal cation and its corresponding neutral metal atom. In some embodiments, the active salt is an alkali metal salt, an alkaline earth metal salt, or any combination thereof. The active salt may be, for example, a lithium salt, a sodium salt, a magnesium salt, a mixture of lithium salts, a mixture of sodium salts, or a mixture of magnesium salts. Advantageously, the active salt is stable towards an alkali metal or alkaline earth metal anode. Exemplary salts include, but are not limited to, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(oxalato)borate (LiBOB), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), sodium bis(oxalato)borate (NaBOB), LiPF₆, LiAsF₆, LiBF₄, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, lithium difluoro oxalato borate anion (LiDFOB), LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, and combinations thereof. In some embodiments, the salt is LiFSI, LiTFSI, NaFSI, NaTFSI, or any combination thereof.

Suitable solvents for use as solvent A include, but are not limited to, certain carbonate solvents, ether solvents, phosphate solvents, dimethyl sulfoxide, water, and mixtures thereof. The solvent A associates with (e.g., solvates or coordinates) cations of the active salt or salt mixture. When prepared as a superconcentrated electrolyte comprising the active salt and the solvent A, solvent-cation-anion aggregates form. Some embodiments of the disclosed superconcentrated electrolytes are stable toward anodes (e.g., a metal or carbon-based anode), cathodes (including ion intercalation and conversion compounds), and current collectors (e.g., Cu, Al) that may be unstable when lower concentration electrolytes are used and/or when other solvents are used. Exemplary solvents for use as solvent A include dimethoxyethane (DME), 1,3-dioxolane (DOL), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl sulfoxide (DMSO), ethyl vinyl sulfone (EVS), tetramethylene sulfone (TMS), also called sulfolane, ethyl methyl sulfone (EMS), ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-vinyl-1,3-dioxolan-2-one (VEC), dimethyl sulfone, ethyl methyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, methylene ethylene carbonate (MEC), 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, water, and combinations thereof. In some embodiments, solvent A is DMC, DME, DOL, EMC, or a combination thereof. In certain embodiments, solvent A is DMC, DME, DOL, or a combination thereof. In one embodiment, solvent A is DMC. In an independent embodiment, solvent A is DME. In another independent embodiment, solvent A is a combination of DME and DOL. In yet another embodiment, solvent A is EMC. In still another independent embodiment, solvent A is DMSO. In yet another independent embodiment, solvent A is water.

As discussed previously, in a superconcentrated electrolyte it is advantageous to have few, substantially no, or no free solvent molecules, i.e., solvent molecules that are not associated with cations of the active salt or salt mixture. The concentration of the active salt may be selected to minimize the number of free solvent A molecules in the electrolyte. Because more than one molecule of solvent A may be associated with each cation of the active salt and/or because more than cation of the active salt may be associated with each molecule of solvent A, the molar ratio of active salt to solvent A may not be 1:1. In some embodiments, a molar ratio of the active salt to the solvent A (moles salt/moles solvent A) is within a range of from 0.33 to 1.5, such as within a range of from 0.5 to 1.5, 0.67 to 1.5, 0.8 to 1.2, or 0.9 to 1.1.

The diluent is a component in which the active salt is insoluble or has poor solubility, i.e., a solubility at least 10× less than the active salt's solubility in the solvent A. For instance, if the salt has a solubility of 5 M in the solvent A, the diluent is selected such that the salt has a solubility of less than 0.5 M in the diluent. In some embodiments, the active salt has a solubility in the solvent A that is at least 10 times, at least 15 times, at least 20 times, at least 25 times, at least 30 times, at least 40 times, or at least 50 times greater than the active salt's solubility in the diluent. The diluent is selected to be stable with the anode, cathode, and current collectors at low active salt concentrations (e.g., ≤3 M) or even without the active salt. In some embodiments, the diluent is selected to have a low dielectric constant (e.g., a relative dielectric constant ≤7) and/or low donor number (e.g., a donor number≤10). Advantageously, the diluent does not disrupt the solvation structure of solvent A-cation-anion aggregates and is considered inert because it is not interacting with the active salt. In other words, there is no significant coordination or association between the diluent molecules and the active salt cations. The active salt cations remain associated with solvent A molecules. Thus, although the electrolyte is diluted, there are few or no free solvent A molecules in the electrolyte.

In some embodiments, the diluent comprises an aprotic organic solvent. In certain embodiments, the diluent is a fluorinated solvent having a wide electrochemical stability window (e.g., >4.5 V), such as a hydrofluoroether (HFE) (also referred to as a fluoroalkyl ether). HFEs advantageously have low dielectric constants, low donor numbers, reductive stability with the metal of the active salt (e.g., lithium, sodium, and/or magnesium), and/or high stability against oxidation due to the electron-withdrawing fluorine atoms. Exemplary fluorinated solvents include, but are not limited to, 1,1,2,2-tetrafluoroethyl-2,2,2,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), and combinations thereof. In some embodiments, selecting a nonflammable diluent (e.g., MOFB, EOFB) significantly improves safety of practical rechargeable batteries, such as lithium and sodium batteries.

Flammable:

Nonflammable:

In some embodiments of the disclosed LSEs, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% of the molecules of solvent A are associated (e.g., solvated or coordinated) with cations of the active salt. In certain embodiments, fewer than 10%, such as fewer than 5%, fewer than 4%, fewer than 3%, or fewer than 2% of the diluent molecules are associated with cations of the active salt. The degree of association can be quantified by any suitable means, such as by calculating the peak intensity ratio of solvent molecules associated with cations and free solvent in Raman spectra or by using NMR spectra.

The relative amounts of the solvent A and diluent are selected to reduce the cost of materials for the electrolyte, reduce viscosity of the electrolyte, maintain stability of the electrolyte against oxidation at high-voltage cathodes, improve ionic conductivity of the electrolyte, improve wetting ability of the electrolyte, facilitate formation of a stable solid electrolyte interphase (SEI) layer, or any combination thereof. In one embodiment, a molar ratio of the solvent A to the diluent (moles solvent A/moles diluent) in the electrolyte is within a range of from 0.2 to 5, such as within a range of from 0.2 to 4, 0.2 to 3, or 0.2 to 2. In an independent embodiment, a volumetric ratio of the solvent A to the diluent (L solvent/L diluent) in the electrolyte is within a range of from 0.2 to 5, such as within a range of from 0.25 to 4 or 0.33 to 3. In another independent embodiment, a mass ratio of the solvent A to the diluent (g solvent/g diluent) in the electrolyte is within a range of from 0.2 to 5, such as within a range of from 0.25 to 4 or 0.33 to 3.

Advantageously, certain embodiments of the disclosed LSEs allow significant dilution of the active salt without sacrificing performance of the electrolyte. Due to the interactions between cations of the active salt and molecules of solvent A, the behavior of the electrolyte corresponds more closely to the concentration of the active salt in the solvent A. Because the diluent is present, however, the active salt may have a molar concentration in the electrolyte that is at least 20% less than the molar concentration of the active salt in the solvent A. In certain embodiments, the molar concentration of the active salt in the electrolyte is at least 25% less, at least 30% less, at least 40% less, at least 50% less, at least 60% less, at least 70% less, or even at least 80% less than the molar concentration of the active salt in the solvent A.

In some embodiments, the formation of cation-anion-solvent aggregates also reduces the lowest unoccupied molecular orbital (LUMO) energy of the anions so they can form a stable solid electrolyte interphase (SEI). As described in Example 10, when the LUMOs of the conduction bands are located at the solvent molecules, the solvent molecules are reductively decomposed at the anode, leading to corrosion of the anode and fast capacity degradation upon cycling. In contrast, the lowest energy level of conduction bands of the anions in certain embodiments of the disclosed LSEs is lower than those of the solvent, indicating that the anions instead of the solvent molecules will be decomposed, forming a stable SEI.

In some embodiments, the diluent is miscible with solvent A. In other embodiments, the diluent is immiscible with solvent A, e.g., when solvent A comprises water and the diluent is a fluorinated organic solvent as disclosed herein. When the solvent A and the diluent are immiscible, the electrolyte may not be effectively diluted with the diluent.

Accordingly, in some embodiments, when the diluent is immiscible with solvent A, the electrolyte further comprises a bridge solvent. The bridge solvent has a different chemical composition than either the solvent A or the diluent. The bridge solvent is selected to be miscible with both solvent A and the diluent, thereby enhancing the practical miscibility of solvent A, and the diluent. In some embodiments, molecules of the bridge solvent are amphiphilic, including both a polar end or moiety, and a non-polar end or moiety, such that molecules of the bridge solvent will associate both with molecules of solvent A and molecules of the diluent as shown in FIG. 3, thereby improving the miscibility between solvent A, and the diluent. Exemplary bridge solvents include, but are not limited to, acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), and combinations thereof.

Exemplary solvent, diluent, and, in some instances, bridge solvent combinations include EMC-BTFE, EMC-TTE, DMC-BTFE, DME-BTFE, DME-TTE, DOL-DME-TTE, DMSO-TTE, H₂O-DMC-TTE, H₂O-PC-TTE, H₂O-AN-TTE, and H₂O-DMSO-TTE. In some embodiments, the active salt is LiFSI, LiTFSI, NaFSI, or NaTFSI.

In some examples, the active salt is LiFSI or NaFSI, the solvent is DMC, DME, or EMC, and the diluent is TTE or BTFE. In certain examples, the salt is LiTFSI or NaTFSI, the solvent is DMSO or a mixture of DME and DOL, and the diluent is TTE. In another independent embodiment, the salt is LiTFSI or NaTFSI, the solvent is H₂O, the diluent is TTE, and the bridge solvent is DMC, propylene carbonate (PC), acetonitrile (AN), or DMSO. Exemplary electrolytes include, but are not limited to, LiFSI/DMC-BTFE, LiFSI/DME-BTFE, LiFSI/DME-TTE, LiFSI/EMC-BTFE, LiFSI/EMC-TTE, NaFSI/DME-TTE, NaFSI/DME-BTFE, NaFSI/DMC-BTFE, NaFSI/EMC-BTFE, NaFSI/EMC-TTE, LiTFSI/DMSO-TTE, LiTFSI/DME-DOL-TTE, NaTFSI/DMSO-TTE, NaTFSI/DME-DOL-TTE, LiTFSI/H₂O-DMC-TTE, LiTFSI/H₂O-PC-TTE, LiTFSI/H₂O-AN-TTE, LiTFSI/H₂O-DMSO-TTE, NaTFSI/H₂O-DMC-TTE, NaTFSI/H₂O-PC-TTE, NaTFSI/H₂O-AN-TTE, NaTFSI/H₂O-DMSO-TTE.

III. BATTERIES

Embodiments of the disclosed LSEs are useful in batteries (e.g., rechargeable batteries), sensors, and supercapacitors. Suitable batteries include, but are not limited to, lithium metal batteries, lithium ion batteries, lithium-sulfur batteries, lithium-oxygen batteries, lithium-air batteries, sodium metal batteries, sodium ion batteries, sodium-sulfur batteries, sodium-oxygen batteries, sodium-air batteries, magnesium ion batteries, including aqueous lithium and sodium batteries.

In some embodiments, a rechargeable battery comprises an LSE as disclosed herein, a cathode, an anode, and optionally a separator. FIG. 4 is a schematic diagram of one exemplary embodiment of a rechargeable battery 100 including a cathode 120, a separator 130 which is infused with an electrolyte (i.e., a LSE), and an anode 140. In some embodiments, the battery 100 also includes a cathode current collector 110 and/or an anode current collector 150.

The current collectors can be a metal or another conductive material such as, but not limited to, nickel (Ni), copper (Cu), aluminum (Al), iron (Fe), stainless steel, or conductive carbon materials. The current collector may be a foil, a foam, or a polymer substrate coated with a conductive material. Advantageously, the current collector is stable (i.e., does not corrode or react) when in contact with the anode or cathode and the electrolyte in an operating voltage window of the battery. The anode and cathode current collectors may be omitted if the anode or cathode, respectively, are free standing, e.g., when the anode is metal or a free-standing film comprising an intercalation material or conversion compound, and/or when the cathode is a free-standing film. By “free-standing” is meant that the film itself has sufficient structural integrity that the film can be positioned in the battery without a support material.

In some embodiments, the anode is a metal (e.g., lithium, sodium), an intercalation material, or a conversion compound. The intercalation material or conversion compound may be deposited onto a substrate (e.g., a current collector) or provided as a free-standing film, typically, including one or more binders and/or conductive additives. Suitable binders include, but are not limited to, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, ethylene oxide polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, epoxy resin, nylon, and the like. Suitable conductive additives include, but are not limited to, carbon black, acetylene black, Ketjen black, carbon fibers (e.g., vapor-grown carbon fiber), metal powders or fibers (e.g., Cu, Ni, Al), and conductive polymers (e.g., polyphenylene derivatives). Exemplary anodes for lithium batteries include, but are not limited to, Mo₆S₈, TiO₂, V₂O₅, Li₄Mn₅O₁₂, Li₄Ti₅O₁₂, C/S composites, and polyacrylonitrile (PAN)-sulfur composites. Exemplary anodes for sodium batteries include, but are not limited to NaTi₂(PO₄)₃; TiS₂, CuS, FeS₂, NiCo₂O₄, Cu₂Se, and Li_(0.5)Na_(0.5)Ti₂(PO₄)₃.

Exemplary cathodes for lithium batteries include, but are not limited to, Li-rich Li_(1+w)Ni_(x)Mn_(y)Co_(z)O₂ (x+y+z+w=1, 0≤w≤0.25), LiNi_(x)Mn_(y)Co_(z)O₂ (NMC, x+y+z=1), LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ (NCA), LiNi_(0.5)Mn_(1.5)O₄ spinel, LiMn₂O₄ (LMO), LiFePO₄ (LFP), Li_(4−x)M_(x)Ti₅O₁₂ (M=Mg, Al, Ba, Sr, or Ta; 0≤x≤1), MnO₂, V₂O₅, V₆O₁₃, LiV₃O₈, LiM^(C1) _(x)M^(C2) _(1−x)PO₄ (M^(C1) or M^(C2)=Fe, Mn, Ni, Co, Cr, or Ti; 0≤x≤1), Li₃V_(2−x)M¹ _(x)(PO₄)₃ (M¹=Cr, Co, Fe, Mg, Y, Ti, Nb, or Ce; 0≤x≤1), LiVPO₄F, LiM^(C1) _(x)M^(C2) _(1−x)O₂ ((M^(C1) and M^(C2) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1), LiM^(C1) _(x)M^(C2) _(y)M^(C3) _(1−x−y)O₂ ((M^(C1), M^(C2), and M^(C3) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1; 0≤y≤1), LiMn_(2−y)X_(y)O₄ (X=Cr, Al, or Fe, 0≤y≤1), LiNi_(0.5−y)X_(y)Mn_(1.5)O₄ (X=Fe, Cr, Zn, Al, Mg, Ga, V, or Cu; 0≤y<0.5), xLi₂MnO₃.(1−z)LiM^(C1) _(y)M^(C2) _(z)M^(C3) _(1−y−z)O₂ (M^(C1), M^(C2), and M^(C3) independently are Mn, Ni, Co, Cr, Fe, or mixture thereof; x=0.3-0.5; y≤0.5; z≤0.5), Li₂M²SiO₄ (M²=Mn, Fe, or Co), Li₂M²SO₄ (M²=Mn, Fe, or Co), LiM²SO₄F (M²=Fe, Mn, or Co), Li_(2−x)(Fe_(1−y)Mn_(y))P₂O₇ (0≤y≤1), Cr₃O₈, Cr₂O₅, a carbon/sulfur composite, or an air electrode (e.g., a carbon-based electrode comprising graphitic carbon and, optionally, a metal catalyst such as Ir, Ru, Pt, Ag, or Ag/Pd). In an independent embodiment, the cathode may be a lithium conversion compound, such as Li₂O₂, Li₂S, or LiF.

Exemplary cathodes for sodium batteries include, but are not limited to, NaFePO₄, Na₂FePO₄F, Na₂FeP₂O₇, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, NaVPO₄F, NaVPOPOF, Na_(1.5)VOPO₄F_(0.5), NaCo₂O₄, Na₂Ti₃O₇, and Na_(x)MO₂ where 0.4<x≤1, and M is a transition metal or a mixture of transition metals (e.g., NaCrO₂, NaCoO₂, Na_(x)CoO₂ (0.4≤x≤0.9), Na_(2/3)Ni_(1/3)Mn_(2/3)O₂, Na_(2/3)Fe_(1/2)Mn_(1/2)O₂, Na_(2/3)Ni_(1/6)Co_(1/6)Mn_(2/3)O₂, NaNi_(1/3)Fe_(1/3)Mn_(1/3)O₂, NaNi_(1/3)Co_(1/3)O₂, NaNi_(1/2)Mn_(1/2)O₂, Prussian white analogue cathodes (e.g., Na₂MnFe(CN)₆ and Na₂Fe₂(CN)₆), Prussian blue analogue (PBA) cathodes (Na_(2−x)M_(a)[M_(b)(CN)₆]_(1−y).nH₂O, wherein M_(a) and M_(b) independently are Fe, Co, Ni, or Cu, x=0 to 0.2, y=0 to 0.2, n=1 to 10). Other sodium intercalation materials include Na₄Ti₅O₁₂, Fe₃O₄, TiO₂, Sb₂O₄, Sb/C composite, SnSb/C composite, BiSb/C composite, and amorphous P/C composite. In an independent embodiment, the cathode is a sodium conversion compound in which sodium displaces another cation, such as FeSe, CuWO₄, CuS, CuO, CuCl, or CuCl₂.

Exemplary cathodes for magnesium batteries include, but are not limited to, zirconium disulfide, cobalt (II,III) oxide, tungsten selenide, V₂O₅, molybdenum-vanadium oxide, stainless steel, Mo₆S₈, Mg₂Mo₆S₈, MoS₂, Mo₆S_(8−y)Se_(y) where y=0, 1, or 2, Mg_(x)S₃O₄ where 0<x<1, MgCoSiO₄, MgFeSiO₄, MgMnSiO₄, V₂O₅, WSe₂, sulfur, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)/graphene, MnO₂/acetylene black, and carbyne polysulfide.

The separator may be glass fiber, a porous polymer film (e.g., polyethylene- or polypropylene-based material) with or without a ceramic coating, or a composite (e.g., a porous film of inorganic particles and a binder). One exemplary polymeric separator is a Celgard® K1640 polyethylene (PE) membrane. Another exemplary polymeric separator is a Celgard® 2500 polypropylene membrane. Another exemplary polymeric separator is a Celgard® 3501 surfactant-coated polypropylene membrane. The separator may be infused with an electrolyte, as disclosed herein.

In some embodiments, a battery includes a lithium metal anode, a cathode suitable for a lithium battery as disclosed above, a separator, and a LSE comprising (i) an active salt selected from LiFSI, LiTFSI, or a combination thereof, (ii) a solvent selected from DMC, DME, DOL, DEC, DMSO, EMC, water, or a combination thereof, and (iii) a diluent selected from TTE, BTFE, TFTFE, MOFB, EOFB, or any combination thereof. In certain embodiments where the solvent and diluent are immiscible, the LSE further comprises a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent. The bridge solvent may be selected from acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), or any combination thereof. In certain embodiments, the cathode comprises LiNi_(x)Mn_(y)Co_(z)O₂ (NMC), sulfur/carbon, or an air electrode.

In some embodiments, a battery includes a sodium metal anode, a cathode suitable for a sodium battery as disclosed above, a separator, and a LSE comprising (i) an active salt selected from NaFSI, NaTFSI, or a combination thereof, (ii) a solvent selected from DMC, DME, DOL, DEC, DMSO, EMC, water, or a combination thereof, and (iii) a diluent selected from BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof. In certain embodiments where the solvent and diluent are immiscible, the LSE further comprises a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent. The bridge solvent may be selected from acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), or any combination thereof. In one embodiment, the cathode is Na₃V₂(PO₄)₃.

In some embodiments, a battery including a LSE as disclosed herein has a performance equal to, or even better than, a comparable battery including a conventional superconcentrated electrolyte. For instance, the battery including the LSE may have a CE greater than or equal to a comparable battery including a concentrated electrolyte. In some embodiments, the battery has a CE≥95%, such as ≥96%, ≥97%, ≥98%, ≥99%, or even≥99.5%. The battery also may have a greater discharge capacity and/or cycling stability compared to a battery including a conventional superconcentrated electrolyte. In some embodiments, the LSE provides dendrite-free plating on the anode at high current densities (e.g., 0.5-10 mA cm⁻²) with CE greater than 99%. Embodiments of batteries including LSEs as disclosed herein demonstrate stable cycling performance (e.g., as evidenced by a stable CE and/or specific capacity) over a period of at least 10 cycles, at least 25 cycles, at least 50 cycles, at least 75 cycles, at least 100 cycles, at least 200 cycles, or at least 300 cycles. For example, the battery may demonstrate stable cycling performance for 10-500 cycles, such as 25-500 cycles, 50-500 cycles, 100-500 cycles, 200-500 cycles, or 300-500 cycles. In one example, Li∥NMC batteries including an electrolyte comprising 1.2 M LiFSI/DMC-BTFE (DMC:BTFE 1:2 by mole) were demonstrated to undergo dendrite-free lithium plating with long-term stable cycling (at least 300 cycles) with a high CE of 99.3-99.9% and >95% retention even when charged to a high voltage of 4.3 V with fast charging (1C, i.e. 2 mA cm⁻²) (Example 2).

Such excellent performance is not achievable with a conventional 1.2 M LiFSI/DMC electrolyte or low-concentration LiPF₆/carbonate solvents, or even with highly concentrated 5.5 M LiFSI/DMC electrolytes. With 1.2 M LiFSI/DMC electrolyte or low-concentration LiPF₆/carbonate solvents, the Li metal cycling efficiency is very low (<50%) and significant overpotential arises for stripping within only couples of cycles. These results indicate that a large amount of Li deposited on the Cu substrate cannot be removed during the stipping process. This problem can be ascribed to the parasitic side-reactions between Li and electrolyte, and the consequent isolation of Li domains by resistive solid electrolyte interphase (SEI) layers.

In contrast, stable Li electrochemcial plating/stripping profiles with CE as high as 99.2% can be obtained in a superconcentrated 5.5 M LiFSI/DMC electrolyte. Dilution of a concentrated LiFSI/DMC electrolyte with BTFE, which is miscible with DMC but does not solvate Li⁺ ions, gives rise to the formation of a LSE and exhibits good stability towards both Li metal anodes and LiNi_(x)Mn_(y)Co_(z)O₂ (NMC) cathodes at high voltage. In addition, synergistic effects arising from lower viscosity and higher conductivity of BTFE-diluted superconcentrated LiFSI/DMC electrolytes also contribute to the superior electrochemical performances of lithium metal batteries including certain embodiments of the disclosed LSEs. In some examples, LiFSI/DMC-BTFE electrolytes exhibited average CEs of 99.3-99.5%.

In summary, certain embodiments of the disclosed LSEs are cost-effective, enable dendrite-free plating, provide high CE, and/or greatly enhance fast charging and/or stable cycling of batteries. For example, 1.2 M LiFSI/DMC-BTFE enabled dendrite-free Li plating a high CE of ˜99.3%, and provided enhanced fast-charging (2 mA cm⁻²) and stable cycling of Li∥NMC batteries at high voltage cutoff at 4.3 V. Without wishing to be bound by a particular theory of operation, the advantages of certain embodiments of the disclosed LSEs are due to the ‘inert’ nature of the hydrofluoroether diluent that does not break the localized salt/solvent high concentration solvation structures, but plays an important role in improving the interfacial stablity of the metal anode. This outstanding performance is even superior over than that achieved in superconcentrated electrolytes, e.g., 5.5 M LiFSI/DMC. Similar beneficial results are seen with other embodiments of the disclosed LSEs and with other battery types, such as lithium-sulfur batteries, lithium-oxygen batteries, lithium-air batteries, sodium metal batteries, sodium ion batteries, sodium-air batteries, sodium-sulfur batteries, sodium-oxygen batteries, and magnesium ion batteries, including aqueous lithium and sodium batteries.

IV. EXAMPLES

Materials

Lithium hexafluorophosphate (LiPF₆), dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) (all in battery-grade purity) were obtained from BASF Corporation (Florham Park, N.J.). Lithium bis(fluorosulfonyl)imide (LiFSI) was obtained from Nippon Shokubai (Japan) and used after drying at 120° C. in vacuum for 24 h. The electrolytes were prepared by dissolving the desired amount of salt into the solvents. Li chips were purchased from MTI Corporation (Richmond, Calif.). Cu and Al foils were purchased from All Foils (Strongsville, Ohio). The NMC (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) cathode electrode was prepared in the Advanced Battery Facility (ABF) located at Pacific Northwest National Laboratory by mixing NMC, super C carbon, polyvinylidene fluoride (PVDF) binder at a mass ratio of 96:2:2 in N-Methyl-2-pyrrolidone (NMP) and coating the slurry onto Al foil. The electrode laminates were punched into discs (1.27 cm²) after being pre-dried and further dried at ˜75° C. under vacuum for 12 h. The materials were stored and handled in an MBraun LABmaster glove box (Stratham, N.H.) with an argon atmosphere (O₂<0.1 ppm and H₂O<0.1 ppm).

Material Characterizations

Morphology observations and EDS measurements of cycled electrodes were performed on a FEI Quanta or a Helios focused ion beam scanning electron microscope (SEM) 9 Hillsboro, Oreg.) at 5.0 kV. For sample preparation, the cycled electrodes were soaked in pure DMC for 10 min and then rinsed with pure DMC at least 3 times to eliminate remaining electrolytes, and finally dried under vacuum. The cross-sections were obtained by using a razor blade to cut the electrodes. The XPS was performed with a Physical Electronics Quantera scanning X-ray microprobe (Chanhassen, Minn.), which was outfitted with a monochromatic Al Kα X-ray source (1,486.7 eV) for excitation. To avoid side reactions or electrode contamination with ambient oxygen and moisture, samples from cycled electrodes were transported from the glovebox to the SEM and XPS instruments in a hermetically sealed container protected by argon gas. Raman spectra were collected using a Raman spectrometer (Horiba LabRAM HR) with sub-1 cm⁻¹ spectral resolution, with all the samples sealed in a test glass tube. The viscosity (η) of the electrolytes was measured on a Brookfield DV-II+ Pro Viscometer (Middleboro, Mass.) at temperatures of 5° C. and 30° C.

The diffusion coefficients of Li⁺ cation, FSI⁻ anion and solvent molecules (DMC and BTFE) denoted to D_(Li), D_(FSI), D_(DMC)/D_(BTFE) were measured by using the ⁷Li, ¹⁹F, and ¹H pulsed field gradient (PFG) NMR, respectivley, in the temperature range of 0˜50° C. on a 600 MHz NMR spectrometer (Agilent Tech, Santa Clara, Calif.) equipped with a 5 mm liquid NMR probe (Doty Scientific, Columbia, S.C.), with a maximum gradient strength of ˜31 T m⁻¹. Bipolar pulsed gradients stimulated echo sequence (Dbppste in VNMRJ, vender supplied sequence) with a 90 degree pulse lengths of 5.5, 7 and 12 μs were employed at Larmor frequencies of 599.8, 564.3, and 233.1 MHz for ¹H, ¹⁹F and ⁷Li PFG-NMR, respectively. The echo profile S(g) obtained as a function of gradient strength (g) were fit with the Stejskal-Tanner equation [1] (Stejskal et al., J. Chem. Phys. 1965, 42:288-292): S(g)=S(0)exp[−D(γδg)²(Δ−δ/3)]  [1] where S(g) and S(0) are peak intensities at a gradient strength of g and 0, respectively, D is diffsuion coeffcient, γ is the gyromagnetic ratio of observing nuclei, δ and Δ are the gradient pulse length and the duration of the two gradient pulses, respectively. δ and Δ were fixed at 2 ms and 30 ms for all measurements and varied g values to obtain a sufficient decay in the echo profiles. Electrochemical Measurements

Ionic conductivity was measured using BioLogic MCS 10 Fully Integrated Multichannel Conductivity Spectroscopy with a cell made of two parallel Pt electrodes over a temperature range of 5 to 30° C. The conductivity cell constants were pre-determined using conductivity standard solution from Okalon Inc.

Electrochemical cycling tests were carried out using CR2032-type coin cells of a two-electrode configuration with constant current mode on battery testers (Land BT2000 and Arbin BT-2000, Arbin Instruments, College Station, Tex.) at 30° C. in environmental chambers. Coin cells (Li∥Cu, Li∥Li and Li∥NMC) were assembled in the glove box with a Li chip as both the counter and reference electrode. Celgard® 2500 polypropylene membrane was used as the separators except for cells with LiFSI/DMC electrolytes for which Celgard® 3501 surfactant-coated polypropylene membranes were used to ensure good wetting due to the wettability issue with Celgard® 2500 membrane. To standardize the testing, 200 μL of electrolyte (excess amount) was added in each coin cell despite some spilling during cell crimping process. For Li∥Cu cells, the effective area of the Cu foil for Li deposition was 2.11 cm² (diameter 1.64 cm). During each cycle, a desired amount of Li metal was deposited on the Cu substrate at various current densities and then stripped until the potential reached 1.0 V vs Li/Li⁺. Li|Li symmetric cells were assembled with Li metal used as the working and counter electrodes. All the Li∥NMC batteries were assembled using Al-clad coin cell cans for the cathode part to eleminate the corrosion of stainless steel can and the side effects at high voltage, and were tested between 2.7 and 4.3 V. 1C is equal to 160 mA g⁻¹ (which is ˜2 mA cm⁻²) of active NMC materials.

Linear sweep voltammetry (LSV) studies of the electrolyte solutions were conducted in a coin cells using a 2.11 cm² Al foil disk as working electrode on a CHI660C workstation (CH Instruments, Inc., Austin, Tex.) with a scan rate of 0.2 mV/s from OCV (open circuit voltage) to 6 V.

Coulombic Efficiency (CE) Measurement

The following protocol was used to measure average CE of Li metal anode using a Li∥Cu coin cell: 1) one initial formation cycle with Li plating of 5 mAh/cm² on Cu and stripping to 1 V; 2) Plate at 5 mAh/cm² Li on Cu as Li reservoir; 3) repeatedly strip/plate Li with 1 mAh/cm² (or strip to 1 V if over-potential>1 V is needed to strip Li with 1 mAh/cm²) for 9 (n=9) cycles; 4) Strip all Li to 1 V. Current: 0.5 mA/cm². Avg. CE is calculated by dividing the total stripping capacity by the total plating capacity based on the following formula:

${CE}_{{avg}.} = {\frac{{nQ}_{{cycle},{strip}} + Q_{strip}}{{nQ}_{{cycle},{plate}} + Q_{reservoir}} \times 100}$ where n is the cycle number at the charge capacity of Q_(cycle,strip) and discharge capacity of Q_(cycle,plate), Q_(strip) is the charge capacity during the final stripping, and Q_(reservior) is the amount of deposition lithium during step 2. MD Simulations

First-principles density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations were used to characterize the DMC-LiFSI solvation structure in localized superconcentrated electrolyte. All calculations were performed using the Vienna ab Initio Simulation Package (VASP) (Kresse et al., Phys. Rev. B 1996, 54:11169-11186; Kresse et al., Phys. Rev. B 1993, 47:558-563; Kresse et al., Phys. Rev. B 1994, 49:14251-14269). Electron-ion interactions were described by the projector-augmented wave (PAW) pseudopotentials with the cutoff energy of 400 eV (Blochl, Phys Rev B 1994, 50:17953-17979; Kresse et al., Phys Rev B 1999, 59:1758-1775). The exchange-correlation functional was represented using the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) (Perdew et al., Phys Rev Lett 1996, 77:3865-3868). The exchange-correlation functional with a Gaussian smearing width term of 0.05 eV was used in the calculations of electrolytes and LiFSI salt interacting with Li metal anode surface systems. The Monkhorst-Pack k-point mesh grid scheme (4×4×1) was used to obtain the optimized Li anode surface and adsorption of electrolyte and salt molecules in the ground state. The convergence criteria for electronic self-consistent iteration and ionic relaxation were set to 1×10⁻⁵ eV and 1×10⁻⁴ eV, respectively. AIMD simulations of electrolyte-salt mixtures were performed in the canonical (NVT) ensemble at 303 K. The constant temperature of AIMD simulation systems was controlled using the Nose thermostat method with a Nose-mass parameter of 0.5. The time step of 0.5 fs was used in all AIMD simulations. A Monkhorst-Pack k-point mesh grid scheme (2×2×2) was used in AIMD simulations. The total AIMD simulation time for each electrolyte/salt system was 15 ps. The AIMD trajectory of final 5 ps was used to obtain radial distribution functions of Li—O pairs.

Example 1 Lithium Metal Batteries with LiFSI/EMC-BTFE and LiFSI/EMC-TTE Electrolytes

The coulombic efficiencies (CEs) of Li∥Cu cells using different superconcentrated Li salts in carbonate solvent were evaluated. The solubility of different salts in carbonate solvents, e.g. EMC, is different, as reflected by the Li salt:solvent molar ratios shown in FIG. 5A. For example, the LiPF₆:EMC ratio is ca. 1:2.35 in a saturated LiPF₆/EMC solution, while the LiFSI:EMC ratio could be as high as 1:1.1 in a saturated LiFSI/EMC solution. The Li∥Cu cells were cycled at 1 mA cm⁻², with lithium deposition/stripping areal capacity of 0.5 mAh cm⁻² in each cycle. The CE data in FIG. 5B shows that only the concentrated electrolytes based on LiFSI salt, for example LiFSI/EMC (molar ratio 1:1.1, 8.73 mol kg⁻¹), could enable the reversible Li deposition/stripping with a stabilized CE of ˜97%.

Fluoroalkyl ethers were used to dilute the concentrated LiFSI/EMC (molar ratio 1:1.1, 8.73 mol kg⁻¹) electrolyte. The LiFSI salt was confirmed insoluble in the fluoroalkyl ethers, e.g., TTE. On the other hand, TTE is misable with EMC. Therefore, the fluoroalkyl ethers are considered an ‘inert’ diluent of the electrolyte without participating in the solvation of LiFSI. The electrolyte formulations and the corresponding mole numbers of these electrolytes are shown in Table 1.

TABLE 1 Electrolyte number and formulations Electro- lyte Electrolyte formulation M m number LiFSI:EMC:diluent molar ratio (mol L⁻¹) (mol kg⁻¹) E1 LiFSI:EMC = 1:1.1 4.9 8.73 mol kg⁻¹ E2 LiFSI:EMC:BTFE = 1:1.1:0.55 3.5 4.66 mol kg⁻¹ E3 LiFSI:EMC:TTE = 1:1.1:0.55 4.13 mol kg⁻¹

In E2 and E3, the localized superconcentrated LiFSI/EMC (1:1.1) solvation cation-anion aggregates (AGGs) are well maintained, beneficial for achieving higher CE for reversible Li deposition/stripping. The formation of Li⁺-FSI⁻⁻-solvent aggregates also reduces the lowest unoccupied molecular orbital (LUMO) energy of the FSI⁻ anions so they can be decomposed first to form a stable solid electrolyte interphase (SEI). With adding fluoroalkyl ethers to a EMC: fluoroalkyl ether ratio of 2:1, the concentration of LiFSI/EMC-fluoroalkyl ether is largely decreased, which is calculated to be 4.66 mol kg⁻¹ for LiFSI/EMC-BTFE (2:1) and 4.13 mol kg⁻¹ for LiFSI/EMC-TTE (2:1). As shown in FIG. 6, with the addition of fluoroalkyl ethers, i.e. TTE, BTFE as diluent, the average CEs of Li∥Cu cells from 3^(rd) to 86^(th) cycles are up to 98.95%, which is comparable to that of the parent superconcentrated LiFSI/EMC electrolyte (98.93%).

The cycling performance of Li∥NMC cells with concentrated LiFSI/EMC electrolyte without and with BTFE diluent was investigated. Improved wetting of the Celgard® 2500 separator could be visually observed after addition of BTFE, as presented in FIG. 7. The cells were cycled either at C/3 or 1 C with a charge cutoff voltage of 4.5 V. FIG. 8A shows that the addition of BTFE as diluent largely shortened the rest time required to wet the electrode/separator. Cycling performance result shows that the Li∥NMC cell using BTFE diluted LiFSI/EMC-BTFE (2:1) electrolyte exhibits comparable discharge capacity, cycling stability and CE during cycling at both C/3 and 1 C rates, as shown in FIGS. 8A and 8B, respectively. The result suggests that addition of BTFE or TTE as a diluent does not compromise the oxidative stability of the concentrated electrolyte.

Not all salt/solvent combinations provide such excellent results. LiBF₄ was evaluated in propylene carbonate (PC). The cycling performances of 7.5 mol/kg LiBF₄ in PC and 2.5 mol/kg LiBF₄ in PC-TTE (2:1 v:v) were compared in Li∥Cu cells having a lithium areal deposition capacity of 0.5 mAh/cm². The initial two cycles were performed at 0.2 mA/cm² with further cycles at 1 mA/cm². The results are shown in FIGS. 9A and 9B. The LiBF₄/PC electrolyte had an initial CE of approximately 50%. When diluted with TTE, the electrolyte had an average CE over 50 cycles of less than 50%. This is in sharp contrast to the LiFSI/EMC, LiFSI/EMC-BTFE, and LiFSI-EMC-TTE electrolytes with CE values up to 98.95%. LiTFSI-tetraglyme also produces poor CE results.

Example 2 LiFSI/DMC-BTFE Electrolyte Performance in Lithium Metal Batteries

The concept of dilution and formation of the localized concentrated electrolyte solution structure was proven in DMC solvent-based electrolytes. The electrolyte formulations of various concentrations of LiFSI/DMC-BTFE solutions are listed in Table 2. With addition of BTFE, the concentration of 5.5 M LiFSI/DMC electrolyte was diluted to as low as 1.2 M.

TABLE 2 Electrolyte number, formulations and viscosity. Viscosity Density Molarity Molality mPa s Electrolyte Molar Ratio Mass Ratio g cm⁻³ mol L⁻¹ mol kg⁻¹ 30° C. 5° C. LiFSI/DMC- E11 0.51:1.1:2.2 0.95:0.99:4.0 1.38 1.18 1.02 2.7 4.8 BTFE E10 0.64:1.1:1.65 1.20:0.99:3.0 1.44 1.77 1.60 3.9 9.1 E9 0.75:1.1:1.1 1.40:0.99:2.0 1.48 2.52 2.51 10.2 31.9 E8 0.94:1.1:0.55 1.76:0.99:1.0 1.53 3.83 4.72 45.9 206.4 LiFSI/DMC E7 1.0:1.1 1.87:0.99 1.57 5.49 10.09 180.0 >1200 E6 1.0:2.0 1.87:1.80 1.37 3.74 5.55 13.6 52.8 E5 1.0:9.0 1.87:8.11 1.21 1.21 1.23 1.3 2.6 LiPF₆/EC- E4 2.19:1.0:1.27 1.42:4.0:6.0 1.22 1.0 0.93 3.5 7.0 EMC Li plating/stripping profiles of Li∥Cu cells using 1.0 M LiPF₆/EC-EMC (E4), 1.2 M LiFSI/DMC (E5), 3.7 M LiFSI/DMC (E6), and 5.5 M LiFSI/DMC (E7) are presented in FIGS. 10A-10D, respectively. The profiles were obtained using the following protocol: 1) one initial formation cycle with Li plating of 5 mAh cm⁻² on Cu at 0.5 mA cm⁻²; and then Li stripping at 0.5 mA cm⁻² to 1 V; 2) plate 5 mAh cm⁻² Li on Cu at 0.5 mA cm⁻² as Li reservoir; 3) strip/plate Li with 1 mAh cm⁻² with 20% depth for 9 cycles; 4) strip all Li at 0.5 mA cm⁻² to 1 V. The average CE is calculated by dividing the total Li stripping capacity by the total Li plating capacity. With 1.0 M LiPF₆/EC-EMC electrolyte, as well as the 1.2 M LiFSI/DMC electrolyte, the Li metal cycling efficiency is so low (<50%) that significant overpotential arises for stripping within only couples of cycles. The CEs can reach>99.0% for 3.7 M and 5.5 M LiFSI/DMC.

The average CEs of diluted LiFSI/DMC-BTFE electrolytes were also measured using the same protocol as shown in FIGS. 11A-11D. The BTFE-diluted LSEs exhibited CEs that were even higher than the superconcentrated 5.5 M LiFSI/DMC (CE 99.2%) with CEs ranging from 99.3 to 99.5%. The results illustrate that the dilution with BTFE does not alter the localized superconcentrated structures and minimizes the presence of free DMC solvent molecules, therefore maintaining the high stability of the electrolyte toward Li metal anode during repeated plating/stripping processes.

The evolution of Li plating/stripping profiles and CE during long-term cycling in different electrolytes was also examined by repeated plating/stripping cycles in Li∥Cu cells. The cells were cycled at a current density of 0.5 mA·cm²; the working area of the Cu electrode was 2.11 cm². FIGS. 12A-12D are scanning electron microscopy images of lithium plated onto copper substrates at the 100th cycle (1 mA/cm² to 0.5 mAh/cm²) from 1.0 M LiPF₆/EC-EMC (4:6, w) (FIG. 12A), 5.5 M LiFSI:DMC (1:1) (FIG. 12B), 3.7 M LiFSI/DMC (1:2) (FIG. 12C), and 1.2 M LiFSI/DMC-BTFE (0.51:1.1:2.2) (FIG. 12D) electrolytes. Cells with the highly concentrated 5.5 M LiFSI/DMC and highly diluted 1.2 M LiFSI/DMC-BTFE (1:2) electrolytes were stably cycled for >200 cycles (FIG. 13) with average CE of ˜99%. Due to better conductivity and lower viscosity (FIG. 14, Table 2), the cell polarization during charge-discharge in the diluted electrolyte was much smaller than that in the highly concentrated electrolyte, and this difference became more obvious with increasing current densities (FIGS. 15A, 15B).

Example 3 Lithium Ion Batteries with LiFSI/DMC-BTFE Electrolytes

The stability of the concentrated and BTFE diluted electrolytes at high voltages was first studied in cells with Al as working electrodes via linear sweep voltammetry (LSV) at a scan rate of 0.2 mV/s. The results show that, although there was less stability at high voltage with increasing BTFE concentrations, the BTFE-diluted electrolytes all exhibited an anodic stability to 4.5 V or higher (FIG. 16). FIGS. 17A-17D are photomicrographs showing the morphology of lithium metal after plating on Cu substrates in low and moderate concentration electrolytes of LiFSI in DMC—1.2 M LiFSI/DMC (FIGS. 17A, 17B) and 3.7 M LiFSI/DMC (FIGS. 17C, 17D).

Next, Li∥NMC cells were assembled to evaluate the performance of a dilute 1.2 M LiFSI/DMC-BTFE electrolyte, by using NMC electrodes of high areal capacity of ˜2.0 mAh/cm². Li∥NMC cells with concentrated 5.5 M LiFSI/DMC and conventional 1.0 M LiPF₆/EC-EMC were also assembled and tested for comparison. FIGS. 18A-18D show the long-term cycling performance and corresponding voltage profile evolution of the Li∥NMC cells. After 3 formation cycles at 0.67 mA/cm² (⅓ hour rate), all the cells were subjected to cycling at a high current density of 2.0 mA/cm² (1 hour rate), in order to reveal the stability of these electrolytes with Li metal anodes at this harsh condition. During the formation cycles, the Li∥NMC cells delivered similar specific discharge capacities of ca. 160 mAh g⁻¹ of NMC, corresponding to a norminal areal capacity of 2.0 mAh/cm². When cycled at 2.0 mA/cm², the Li∥NMC cell with the 1.0 M LiPF₆/EC-EMC electrolyte showed a drastic increase of electrode polarization and fast capacity degradation with <30% retention after 100 cycles (FIGS. 18A, 18B), which is attributed to the severe corrostion reactions between Li metal and the electrolyte. When using the concentrated 5.5 M LiFSI/DMC, the stability with Li metal was greatly improved, as reflected by the much higher CE of Li metal itself. However, the Li∥NMC cell with concentrated 5.5 M LiFSI/DMC still showed continuous capacity fading and increase of electrode polarization, retaining ˜70% of its capacity after 100 cycles (FIG. 18C). The unsatisfactory cycling performance of concentrated 5.5 M LiFSI/DMC is attributed to the sluggish electrode reaction kinetics resulted from the high viscosity, low conductivity and poor wetting ability of the superconcentrated electrolyte. In sharp contrast, with the BTFE-diluted 1.2 M LiFSI/DMC-BTFE electrolyte, the significantly improved long-term cycling stability and limited increase of electrode polarization could be achieved (FIG. 18D). The cell shows a high capacity retention of >95% after 300 cycles at high current density of 2.0 mA/cm², which is among the best performances ever reported for Li metal batteries. This finding indicates that the LSE with a low Li salt concentration could enable the fast charging and stable cycling of Li metal batteries (LMBs).

The rate performances of LiIINMC cells were evaluated to determine the electrochemical reaction kinetics of the BTFE diluted electrolyte. The rate capability was tested using two charge/discharge protocols, i.e., (i) charge at same C/5 and discharge at increasing rate; (ii) charge at increasing rate and discharge at same C/5. As shown in FIGS. 19 and 20, the Li∥NMC cell with BTFE-diluted electrolyte (1.2 M LiFSI/DMC-BTFE) showed superior charging and discharging capability as compared to the concentrated 5.5 M LiFSI/DMC electrolyte and the baseline 1 M LiPF₆ dilute electrolyte. In particular, with protocol (i), when discharging at 5 C (i.e. 10 mA/cm²), the cell using 1.2 M LiFSI/DMC-BTFE electrolyte delivered a high discharge capacity of 141 mAh/g, significantly higher than 116 mAh/g for the concentrated 5.5 M LiFSI/DMC electrolyte and 68 mAh/g for the 1 M LiPF₆ dilute electrolyte. The enhanced rate capability of 1.2 M LiFSI/DMC-BTFE electrolyte as compared to concentrated 5.5 M LiFSI/DMC electrolyte is ascribed to the reduced viscosity, increased conductivity, improved electrode/separator wetting, as well as the improved interfacial reaction kinetics.

Example 4 Dendrite-Free Deposition on Li Metal Anode

To gain insight into the superior electrochemical properties of LSE (1.2 M LiFSI/DMC-BTFE), the morphology features of Li deposited in different electrolytes were evaluated. The current density was 1.0 mA/cm² and the deposition capacity was 1.5 mAh/cm². FIGS. 21A-21F show the cross-section and surface morphologies of Li films deposited on Cu substrates in different electrolytes at a current desity of 1 mA/cm². The electrolytes were as follows: 1.0 M LiPF₆/EC-EMC (FIGS. 21A, 21B), 5.5 M LiFSI/DMC (FIGS. 21C, 21D), and 1.2 M LiFSI/DMC-BTFE (1:2) (FIGS. 21E, 21F). The plating of Li metal from the LiPF₆ electrolyte resulted in a highly porous/loose structure with extensive dendritic Li (FIGS. 21A, 21B). Dendritic Li deposition were also observed in both low (1.2 M) and moderate (3.7 M) concentrated LiFSI/DMC electrolytes as previously shown in FIGS. 17A-17D. In contrast, nodule-like Li deposits without dendrite formation were obtained in both highly concentrated 5.5 M LiFSI/DMC and diluted low concentration 1.2 M LiFSI/DMC-BTFE electrolytes (FIGS. 21C-21F). Compared to that in the concentrated electrolyte, larger primary Li particles (avg. ˜5 μm) and a denser deposit layer (˜10 μm, close to the theoretical thickness of 1.5 mAh cm⁻² (˜7.2 μm) of bulk Li) were found in the LSE electrolyte (1.2 M LiFSI/DMC-BTFE). Moreover, with increasing current densities (2, 5, and 10 mA/cm²), the Li deposits maintained a nodule-like nature in the 1.2 M LiFSI/DMC-BTFE electrolyte despite a slight decrease in particle size (FIGS. 22A-22C). Formation of nodule-like Li deposits of large particle size can significantly mitigate the interfacial reactions with the electrolyte, and reduce the risk of Li penetration through the separator, hence improving the cycle life and safety of LMBs using LSE (dilute 1.2 M LiFSI/DMC-BTFE). The dilute electrolyte also produced a more stable SEI layer than the highly concentrated 5.5 M LiFSI/DMC. In addition, deposition of high density Li is beneficial for reducing the volume variation of the LMBs during charge/discharge processes, and is highly desirable for the development of LMBs.

Example 5 Lithium Metal Batteries with LiFSI/DME-BTFE and LiFSI/DME-TTE Electrolytes

Dilution of concentrated electrolytes also works well with ether-based electrolytes, such as DME. Exemplary electrolyte formulations are shown in Table 3. With addition of BTFE or TTE, the concentration of 4 M LiFSI/DME electrolyte was diluted to 2 M or 1 M.

TABLE 3 Electrolyte number and formulations Electrolyte Concen- number Electrolyte formulation tration E12 4M LiFSI/DME (LiFSI:DME 1:1.4) 4 mol L⁻¹ E13 LiFSI/DME + BTFE (DME:BTFE = 3:5, v:v) 2 mol L⁻¹ E14 LiFSI/DME + TTE (DME:TTE = 3:5, v:v) 2 mol L⁻¹ E15 LiFSI/DME + BTFE (DME:BTFE = 3:8, v:v) 1 mol L⁻¹ E16 LiFSI/DME + TTE (DME:TTE = 3:8, v:v) 1 mol L⁻¹

FIG. 23 shows the CE of Li∥Cu cells using concentrated LiFSI/DME electrolyte and those with TTE or BTFE diluent. All cells demonstrated very similar CE in the first cycle and during the long-term cycling. When the concentration of LiFSI/(DME+BTFE) and LiFSI/(DME+TTE) was diluted to 2M with the addition of fluoroalkyl ethers, i.e. TTE, BTFE as diluent, the average CEs of Li∥Cu cells were 98.83% and 98.94% for TTE and BTFE diluted electrolyte, which is comparable to or even superior over that of the parent concentrated LiFSI/DME (98.74%). Even when the concentration of LiFSI/(DME+BTFE) and LiFSI/(DME+TTE) was diluted to 1M with the addition of TTE, BTFE as diluent, the average CEs of Li∥Cu cells were 98.90% and 98.94% and 98.74 for TTE and BTFE diluted electrolyte, which is very comparable to or even superior over that of the parent concentrated LiFSI/DME (98.74%).

The cycling performance for Li∥LFP cells containing concentrated 4 M LiFSI/DME electrolyte without and with TTE or BTFE diluent is shown in FIG. 24. The Li∥LFP coin cells were assembled using LFP cathode (Hydro-Quebec, 1 mAh cm⁻²), lithium metal anode, one piece of polyethylene (PE) separator, and the concentrated ether based electrolyte before and after dilution. Diluting the concentrated LiFSI/DME electrolyte from 4 M to 2 M, the Li∥LFP cells using 2 M LiFSI/(DME+BTFE) electrolyte and 2 M LiFSI/(DME+TTE) electrolyte show similar long-term cycling stability as compared to the cell using parent concentrated 4 M LiFSI/DME electrolyte. Results were obtained at a 1C rate after 3 formation cycles at C/10, in the voltage range of 2.5˜3.7 V.

Example 6 Sodium Metal Batteries with NaFSI/DME-TTE and NaFSI-DME-BTFE Electrolytes

The concept of localized superconcentrated electrolytes also works well in sodium metal batteries. The evaluated electrolyte formulations are shown in Table 4. With addition of TTE, the concentration of the 5.2 M NaFSI/DME electrolyte was diluted to 1.5 M.

TABLE 4 Electrolyte number and formulations used this example Electrolyte Concen- number Electrolyte formulation tration E17 NaFSI/DME (NaFSI:DME, 1:1 , molar ratio) 5.2 mol L⁻¹ E18 NaFSI/DME + TTE (DME:TTE = 1:0.5, 3.0 mol L⁻¹ molar ratio) E19 NaFSI/DME + TTE (DME:TTE = 1:1, 2.3 mol L⁻¹ molar ratio) E20 NaFSI/DME + TTE (DME:TTE = 1:2, 1.5 mol L⁻¹ molar ratio)

For charge/discharge performance testing, Na∥Na₃V₂(PO₄)₃ coin cells were constructed in an argon-filled glove box (MBraun, Inc.) using a Na₃V₂(PO₄)₃ cathode, Na metal as the anode, glass fiber as the separator, and NaFSI/DME electrolyte with and without TTE diluent. The Na₃V₂(PO₄)₃ cathode contains 80% Na₃V₂(PO₄)₃, 10% Super P® carbon black (available, e.g., from Fisher Scientific), and 10% PVDF (polyvinylidene fluoride).

FIGS. 25A and 25B show the initial Na plating/stripping voltage profiles (FIG. 25A) and CE vs. cycle number of Na∥Cu cells with Na deposition areal capacity of 1.3 mAh cm⁻² (FIG. 25B). CE was evaluated as a function of cycle number tested at 0.65 mA cm⁻² after two formation cycles at 0.26 mA cm⁻², with Na deposition areal capacity of 1.3 mAh cm⁻². The initial CEs for NaFSI/DME and NaFSI/DME-TTE electrolytes with 5.2 M, 3.0 M, 2.3 M and 1.5 M NaFSI salt were 94.3%, 96.1%, 94.8% and 96.5%, respectively. During the following cycling, the CEs of the diluted electrolytes were comparable or higher than that of the parent 5.2 M NaFSI/DME electrolyte.

FIGS. 26A, and 26B show the electrochemical performance of Na∥Na₃V₂(PO₄)₃ cells using concentrated 5.2 M NaFSI/DME electrolyte and TTE-diluted NaFSI/DME-TTE electrolytes (2.3 M and 1.5 M) at a rate of C/3. The cells using TTE diluted NaFSI/DME-TTE electrolytes showed similar initial specific discharge capacity of ca. 97 mAh g⁻¹ as compared to concentrated NaFSI/DME electrolyte (FIG. 26B). FIGS. 27A and 27B, respectively, show the charge and discharge capacities of the cells using 5.2 M NaFSI/DME electrolyte and 2.3 M NaFSI/DME-TTE electrolyte.

FIGS. 28A and 28B show the initial Na plating/stripping voltage profiles (FIG. 28A) and CE vs. cycle number of Na∥Cu cells with Na deposition areal capacity of 1.0 mAh cm⁻² (FIG. 28B) with 5.2 M NaFSI/DME, 3.1 M NaFSI/DME-BTFE (1:1), 2.1 M NaFSI/DME-BTFE (1:2), and 1.5 M NaFSI/DME-BTFE (1:3). The ratios in the parentheses are the mole ratios of DME:BTFE. CE was evaluated as a function of cycle number tested at 1 mA cm⁻² after two formation cycles at 0.2 mA cm⁻². As shown in FIG. 28B, the LSE, 2.1 M NaFSI/DME-BTFE (1:2), exhibited stable cycling with a CE of nearly 100% over 200+ cycles.

FIGS. 29A-29C show the electrochemical performance of Na∥Na₃V₂(PO₄)₃ cells using 5.2 M NaFSI/DME and BTFE-diluted NaFSI/DME-BTFE electrolytes—3.1 M NAFSI/DME-BTFE (1:1), 2.1 M NaFSI/DME-BTFE (1:2), and 1.5 M NaFSI/DME-BTFE (1:3). FIG. 29A shows the initial Na plating/stripping voltage profiles. FIG. 29B shows the cycling stability over 100 cycles at rates of C/10 and C/3. FIG. 29C shows the charge and discharge capacities of 2.1 M NaFSI/DME-BTFE (1:2 mol) over 100 cycles at rates of C/10 and C/3. The results indicate that BTFE is a superior diluent for concentrated NaFSI/DME electrolytes.

Example 7 Li—S Batteries with LiTFSI/DOL-DME-TTE Electrolyte

The concept of localized superconcentrated electrolyte also works well for lithium sulfur batteries. The electrolyte formulations evaluated in this example are shown in Table 5. With addition of TTE, the concentration of 3.3 M LiTFSI in DOL-DME (1:1, v:v) electrolyte was diluted to 1.06 M.

TABLE 5 Electrolyte number and formulations Electrolyte Concen- number Electrolyte formulation tration E21 LiTFSI in DOL-DME (1:1, v:v)  1 mol L⁻¹ E22 LiTFSI in DOL-DME (1:1, v:v) 3.3 mol L⁻¹ E23 LiTFSI in DOL-DME-TTE (1:1:9, v:v:v) 1.06 mol L⁻¹ 

For charge/discharge performance testing, Li—S coin cells were assembled in an argon-filled glove box (MBraun, Inc.) using a Ketjen black conductive carbon (KB)/S cathode, lithium metal as the anode, one piece of polyethylene as separator, and the LiTFSI/DOL-DME electrolytes with or without TTE diluent. The KB/S cathode was prepared by coating a slurry containing 80% KB/S composite, 10% PVDF and 10% Super P® conductive carbon. The KB/S composite was prepared by mixing 80% S and 20% KB, followed by heat treatment at 155° C. for 12 hrs.

FIGS. 30A and 30B show the initial Li plating/stripping voltage profiles (FIG. 30A) and CE vs. cycle number (FIG. 30B) of Li∥Cu cells with a Li deposition areal capacity of 1 mAh cm⁻². The results were obtained at 1 mA cm⁻² after 2 formation cycles at 0.2 mA cm⁻² with a Li deposition areal capacity of 1 mAh cm⁻². The superconcentrated 3.3 M LiTFSI/DOL-DME electrolyte showed an initial CE of 91.6%, which was much higher than the 70.1% for the low concentration 1 M LiTFSI/DOL-DME electrolyte. With dilution the 3.3 M LiTFSI/DOL-DME electrolyte with TTE, the localized superconcentrated Li⁺ solvation structure was maintained and the diluted 1.06 M LiTFSI in DOL-DME-TTE (1:1:9, v:v:v) electrolyte showed a higher CE of 96.4% during the initial plating/stripping process.

The electrochemical performance for Li—S cells containing regular low concentration 1 M LiTFSI/DOL-DME, concentrated 3.3 M LiTFSI/DOL-DME electrolyte, and diluted 1.06 M LiTFSI in DOL-DME-TTE electrolyte is shown in FIGS. 31A-31C. FIG. 31A shows the initial charge/discharge voltage profiles. In FIG. 31A, curves A were obtained with 1.0 M LiTFSI/DOL-DME, curves B were obtained with 3.3 M LiTFSI/DOL-DME, and curves C were obtained with 1.06 M LiTFSI/DOL-DME-TTE. The Li—S cells with electrolyte/sulfur ratio of 50 mL/g were cycled at a low current rate of C/10 (168 mA/g). Diluting the concentrated LiTFSI/DOL-DME electrolyte from 3.3 M to 1.06 M, the Li—S cells showed even better long-term cycling stability (FIG. 31B) and higher CE (FIG. 31C) as compared to the cell using parent concentrated 3.3M LiTFSI/DOL-DME electrolyte.

Example 8 Li—O₂ Batteries with LiTFSI/DMSO-TTE Electrolyte

The concept of localized superconcentrated electrolyte was also investigated in lithium oxygen battery. The electrolyte formulations evaluated are listed in Table 6. With addition of TTE, the concentration of 2.76 M LiTFSI in DMSO (LiTFSI:DMSO molar ratio 1:3) electrolyte was diluted to 1.23 M (LiTFSI:DMSO:TTE molar ratio 1:3:3).

TABLE 6 Electrolyte number and formulations Molar ratio of salt Concen- Electrolyte Electrolyte to solvent tration number denotation (LiTFSI:DMSO:TTE) (mol/L) E24 LITFSI-12DMSO 1:12   1M E25 LITFSI-3DMSO 1:3 2.76M E26 LITFSI-3DMSO-3TTE 1:3:3 1.23M

FIG. 32 shows the charge/discharge profiles of Li—O₂ cells using LiTFSI-3DMSO (2.76 M) and LiTFSI-3DMSO-3TTE (1.23 M) electrolytes with limited discharge capacity of 600 mAh g⁻¹ at a current density of 0.1 mA cm⁻². After dilution of LiTFSI concentration from 2.76 M to 1.23 M, the Li—O₂ cells demonstrated similar capacity as those observed in the Li—O₂ cells using the highly concentrated electrolyte. For charge/discharge performance testing, Li—O₂ coin cells were assembled in the argon-filled glove box (MBraun, Inc.). A piece of separator (Whatman glass fiber B) soaked with 200 μL of LiTFSI-DMSO electrolyte with and without TTE diluent was placed between an air electrode disc and a Li metal chip. After crimping, the assembled cells were transferred into PTFE (polytetrafluoroethylene) containers and taken out of the glove box. The PTFE containers were filled with ultrahigh purity O₂. These Li—O₂ cells were allowed to rest for at least 6 h in O₂ atmosphere and then discharge/charge evaluations were performed at room temperature on an Arbin BT-2000 battery tester (Arbin Instruments, College Stations, Tex.) at a current density of 0.1 mA cm⁻².

Example 9 Aqueous Electrolytes with Bridge Solvents

Water based electrolytes with superconcentrated lithium salt LiTFSI (e.g., 21 mol LiTFSI in 1 kg of H₂O) have been reported to widen their electrochemical stability window to about 3.0 V (i.e. 1.9˜4.9 vs. Li/Li⁺). As demonstrated in the above examples, diluting this concentrated water based electrolyte while maintaining the wide electrochemical stability window could be a good strategy to reduce the cost of the electrolyte. However, the fluoroalkyl ethers having low dielectric constants and low donor numbers are not miscible with water. Therefore, the superconcentrated aqueous electrolyte (21 mol LiTFSI in 1 kg of H₂O) was diluted with TTE with the assistance of a ‘bridge’ solvent. The evaluated electrolyte formulations are shown in Tables 7 and 8. The ‘bridge’ solvents included DMC, PC, AN, and DMSO. By fixing the ratio of H₂O:TTE to be 1:1, the optimal content of ‘bridge’ solvent was carefully optimized. The optimal content is different for different solvents, depending on the strength of the interaction between ‘bridge’ solvent and H₂O and TTE diluent.

TABLE 7 Electrolyte number and formulations Electrolyte H₂O-solvent- m M number TTE wt. ratio (mol/kg) (mol/L) E27 H₂O 1 21 5.04 E28 H₂O-DMC-TTE 1:0.85:1 7.37 3.78 E29 H₂O-PC-TTE 1:1.05:1 6.89 3.69 E30 H₂O-AN-TTE 1:0.5:1 8.4 3.87 E31 H₂O-DMSO-TTE 1:1.15:1 6.67 3.69

FIG. 33 shows the cyclic voltammograms of concentrated aqueous electrolyte before and after dilution with TTE with the assistance of different ‘bridge’ solvents, including DMC, PC, AN, and DMSO, using a stainless steel working electrode and counter electrode, and Ag/AgCl as reference electrode at a scan rate of 10 mV s⁻¹. The potential was converted to those versus to Li/Li⁺ redox couple. In comparison, it was found that the dilution with TTE using PC as the ‘bridge’ solvent did not compromise much of the oxidation and reduction stability. However, with a H₂O:TTE ratio of 1:1, the concentration of the LiTFSI/H₂O solution was only diluted from 5.04 M to 3.69 M.

To further dilute the concentrated LiTFSI/H₂O solution, more TTE was used with the addition of increased PC as ‘bridge’ solvent. The ratio between PC:TTE was very close to 1:1 in diluted electrolytes. With the increased addition of PC and TTE, the concentration of the concentrated LiTFSI/H₂O solution was diluted from 5.04 M to 2.92 M.

TABLE 8 Electrolyte number and formulations Electrolyte H₂O-solvent- m M number TTE wt. ratio (mol/kg) (mol/L) E32 H₂O 1 21 5.04 E33 H₂O-PC-TTE 1:0.55:0.5 10.3 4.30 E34 H₂O-PC-TTE 1:1.05:1 6.89 3.69 E35 H₂O-PC-TTE 1:1.5:1.5 5.25 3.27 E36 H₂O-PC-TTE 1:2:2 4.29 2.92

FIGS. 34A and 34B show the first and second cycle cyclic voltammograms, respectively, of concentrated aqueous electrolyte diluted with different amounts of TTE with the assistance of PC. Stainless steel was the working electrode and counter electrode, and Ag/AgCl was the reference electrode; scan rate of 10 mV s⁻¹. The potential was converted to those versus to Li/Li⁺ redox couple. In the first cycle (FIG. 34A), the increased addition of PC-TTE leads to a slight increase in the reduction instability at ˜2.35V (arrow), as reflected by the higher current response during the negative scan. However, dilution of an appropriate amount of PC-TTE improves the reduction stability and oxidation stability, probably due to the formation of an enhanced SEI layer on the working electrode (FIG. 34B). The optimum dilution of the concentrated electrolyte is identified as H₂O:PC:TTE ratio of 1:1.5:1.5.

Example 10 Molecular Simulations

Without wishing to be bound by a particular theory of operation, the exceptional electrochemical performances of LMBs achieved in the BTFE diluted LiFSI/DMC-BTFE electrolytes is believed to stem from its unique highly localized superconcentrated Li salt solvation structure. First-principles density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations were used to characterize the DMC-LiFSI solvation structure in localized superconcentrated electrolyte. All calculations were performed using the Vienna ab Initio Simulation Package (VASP) (Kresse et al., Phys Rev B 1996, 54:11169-11186; Kresse et al., Phys Rev B 1993, 47:558-561; Kresse et al., Phys Rev B 1994, 49:14251-14269). Electron-ion interactions were described by the projector-augmented wave (PAW) pseudopotentials with the cutoff energy of 400 eV (Blochl, Phys Rev B 1994, 50:17953-17979; Kresse et al., Phys Rev B 1999, 59:1758-1775). The exchange-correlation functional was represented using the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) (Perdew et al., Phys Rev Lett 1996, 77:3865-3868). The exchange-correlation functional with a Gaussian smearing width term of 0.05 eV was used in the calculations of electrolytes and LiFSI salt interacting with Li metal anode surface systems. The Monkhorst-Pack k-point mesh grid scheme (4×4×1) was used to obtain the optimized Li anode surface and adsorption of electrolyte and salt molecules in the ground state. The convergence criteria for electronic self-consistent iteration and ionic relaxation were set to 1×10⁻⁵ eV and 1×10⁻⁴ eV, respectively. AIMD simulations of electrolyte-salt mixtures were performed in the canonical (NVT) ensemble at 303 K. The constant temperature of AIMD simulation systems was controlled using the Nose thermostat method with a Nose-mass parameter of 0.5. The time step of 0.5 fs was used in all AIMD simulations. A Monkhorst-Pack k-point mesh grid scheme (2×2×2) was used in AIMD simulations. The total AIMD simulation time for each electrolyte/salt system was 15 ps. The AIMD trajectory of final 5 ps was used to obtain radial distribution functions of Li—O pairs.

FIG. 35 shows the optimized geometries of DMC, BTFE, LiFSI, and pairs of DMC/LiFSI, BTFE/LiFSI in vacuum using VASP. The Li, O, C, H, S, N, and F atoms are colored as magenta, red, gray, white, yellow, blue, and light blue, respectively. Only Γ-centered k-point mesh was used for above calculations. Similar results were also obtained using the Gaussian 09 package with PBE and the 6-311++G(p,d) basis set (Frisch et al., Gaussian 09, 2009, Gaussian Inc., Wallingford, Conn.), Table 10. It was found that LiFSI salt is preferentially coordinated with DMC via the Li—O_(DMC) bond, with an optimized Li—O_(DMC) bond length of 1.85 Å and strong interaction between LiFSI and DMC (−88.7 kJ/mol). On the other hand, the interaction between LiFSI and another electrolyte solvent BTFE is comparatively weak as reflected by the Li—O_(BTFE) bond distance of 2.70 Å, which is much longer than the optimized Li—O_(DMC) bond, and much weaker interaction of −41.4 kJ/mol between LiFSI and BTFE.

As suggested by previous theoretical work of Li bulk and surfaces (Camacho-Forero et al., J Phys Chem C 2015, 119:26828-26839; Doll et al., J Phys Condens Matter 1999, 11:5007-5019), the most stable (100) surface among three low index surface structures, i.e., (100), (110), and (111) crystallographic planes, was used to model the Li anode surface. The periodic Li(100) surface has a p(4×4) super cell with seven atomic Li layers. The optimized structures of DMC, BTFE, LiFSI and the DMC/LiFSI pair on the Li(100) surface are shown in FIGS. 36A-36F.

Bader charge analyses (Henkelman et al., Comput Mater Sci 2006, 36:354-360) were performed to obtain the possible charge transfer between the electrolytes and the salt species upon adsorption. DFT calculated adsorption energies and Bader charges of each species were summarized in Table 9. Compared with DMC, LiFSI and the DMC/LiFSI pair, the interaction between BTFE and the Li anode surface is very weak and barely reductive. This indicates that BTFE molecule is nearly inert and barely being reduced. While DMC and the DMC/LiFSI pair are slightly reduced by obtaining the fractional charges of −0.19 and −0.40 |e|, respectively, implying that both are reduced thus leading to possible decomposition. In summary, BTFE is found to be more stable than DMC and the DMC/LiFSI pair on the Li anode.

To understand the effect of adding the second electrolyte solvent BTFE on the microscopic structures of DMC/LiFSI mixtures, three electrolyte/salt mixture systems, i.e., one binary DMC/LiFSI mixture and two ternary DMC/BTFE/LiFSI mixtures with two different molar ratios, were investigated using AIMD simulations. The initial structure of each liquid electrolyte/salt mixture system was set up by randomly placing the numbers of DMC, BTFE and LiFSI molecules on the basis of experimental densities and molar ratios (concentration). The size of simulation system is 20 Å×20 Å×20 Å. These initial structures were firstly relaxed using a house-made classical molecular dynamics simulation method with the flexible force field (Han et al., J Electrochem Soc 2014, 161:A2042-2053; Soetens et al., J Phys Chem A 1998, 102:1055-1061). Upon quasi-equilibration of the system, a total of 15 ps AIMD simulations were carried out for each mixture system. The snapshots of three electrolyte/salt mixture systems from AIMD summations are shown in FIGS. 37A-37C. FIG. 38 shows the radial distribution functions of Li—O_(DMC) and Li—O_(BTFE) pairs calculated from AIMD simulation trajectories at 303 K. It is clearly seen from the snapshots (FIGS. 37A-37C) of the three electrolyte/salt mixture systems and the radial distribution functions of Li—O_(DMC) and Li—O_(BTFE) pairs (FIG. 38) that all LiFSI salt molecules are closely coordinated with DMC instead of BTFE. The last 5 ps AIMD simulation trajectories were used to calculate the radial distribution functions of Li—O_(DMC) and Li—O_(BTFE) pairs. As shown in FIG. 38, a sharp peak of the Li—O_(DMC) pair is identified at 1.95 Å for all three studied systems, indicating that all LiFSI salts are surrounded by DMC solvent molecules as the first coordination shell. This is due to the strong attractive interaction between DMC and LiFSI. Two small peaks of the Li—O_(BTFE) pair at 4.65 and 5.63 Å are found for two ternary mixture systems with high and low BTFE concentrations. This suggests that BTFE is not coordinated to LiFSI in the two DMC/BTFE/LiFSI mixtures, clearly indicative of the existence of the localized superconcentration of DMC/LiFSI pairs, independent of the concentration of the BTFE diluent molecules.

Raman spectroscopy and NMR spectroscopy were employed to investigate the solvation structures of concentrated LiFSI/DMC electrolyte and BTFE diluted LiFSI/DMC-BTFE electrolytes. FIGS. 39A and 39B show Raman spectra of pure DMC, BTFE, and a DMC-BTFE (2:1) solvent mixture. There is no observable Raman shift in the peak locations of DMC and BTFE in the mixture of DMC and BTFE. This result indicates that there is no significant interaction between DMC and BTFE.

In the Raman spectra of different concentrations of LiFSI/DMC solutions, as presented in FIG. 40A, the dilute LiFSI/DMC (1:9) solution is featured by the dominating O—CH₃ stretching vibration band at ˜920 cm⁻¹ for free DMC solvent molecules and the minor vibration band at ˜940 cm⁻¹ for Li⁺-coordinated DMC. As the LiFSI concentration is increased (higher LiFSI:DMC molar ratio), the free DMC diminishes and disappears to form the contact ion pairs (CIPs, FSI⁻ coordinating to single Li⁺ ion) and aggregates (AGGs, FSI⁻ coordinating to two or more Li⁺ ions). Formation of CIPs and AGGs in concentrated 5.5 M LiFSI/DMC is also proved by the obvious upshift of the FSI⁻ Raman band at 710˜780 cm⁻¹. FIG. 40B shows the Raman spectra of different concentrations of BTFE diluted LiFSI/DMC solutions. With dilution by BTFE, it is found that the Li⁺-coordinated DMC solvation structure is well preserved, without any shift of the vibration band at ˜940 cm⁻¹ being observed. Moreover, the vibration band of BTFE at 830˜840 cm⁻¹ does not change in different LiFSI/DMC-BTFE solutions. The result suggests that the diluent BTFE is not involved in the solvation of Li⁺ cation because of its low dielectric constant and low donor number, confirming the LSE solvation structures. One feature of note is that the dilution with BTFE weakens slightly the association between the Li⁺ cation and FSI-anion, as evidenced by the downshift of the FSI⁻ Raman band (710˜780 cm⁻¹), which would be beneficial for enhancing the Li⁺ ion diffusion and improving the kinetic properties of the LMBs.

The LSE solvation structure is also confirmed by NMR characterization. NMR data (FIG. 41) shows that all diffusion coefficients (Ds) are globally proportional to the inverse of viscosity (η⁻¹) of solutions while its variations are slightly different depending on the ion-ion and ion-solvent interactions as predicted by the Stokes-Einstein theory of diffusion (Pregosin et al., Chemical Reviews 2005, 105:2977-2998)). It is found D_(DMC)>D_(BTFE) in pure DMC, BTFE solvent and their mixture, but with introduction of LiFSI salt, D_(DMC) and D_(BTFE) become smaller and larger than η⁻¹, respectively. This strongly suggests that the Li⁺ cation solvation occurs mainly by DMC molecules, with BTFE interaction to other electrolyte components being quite weak. It also indicates that Li⁺ cation diffusion is enhanced (D_(Li)≥D_(FSI)) by addition of BTFE, which is in line with Raman observation, while is reversed in the LiFSI/DMC electrolytes (D_(Li)≤D_(FSI)).

The stable diffusion ratios D_(Li)/D_(DMC) and D_(FSI)/D_(DMC) suggest that the LSE solvation structure composed of Li⁺ cation, FSI⁻ anion and DMC solvent is not sensitive to the population of BTFE in the LiFSI/DMC-BTFE electrolytes (FIG. 42). The temperature dependent Ds appears to obey the Stokes-Einstein theory of diffusion, D=k_(B)T/6πηr_(s), where D is diffusion coefficient, k_(B) is the Boltzmann constant, T is the absolute temperature, η is the viscosity and r_(s) is the hydrodynamic radius of diffusing molecule. The D_(Li)/D_(DMC) and D_(FSI)/D_(DMC) values are closer to unity and D_(BTFE)/D_(DMC) values are much larger than the unity due to the preferential solvation of Li⁺ cation by DMC molecules. Relatively constant D_(Li)/D_(DMC) and D_(FSI)/D_(DMC) values in DMC:BTFE electrolytes suggests that the solvation structure composed of Li⁺ cation, FSI⁻ anion and DMC is less sensitive to the concentration of LiFSI and the ratio between DMC:BTFE. It can be concluded that the ion-ion and ion-solvent interactions, which depend strongly on the concentration of Li salt, are less likely to change as a function of LiFSI concentration in a DMC:BTFE binary electrolyte system.

To gain insight into the stability of electrolyte components with Li metal, the moderately dilute 3.7 M LiFSI/DMC, SE (5.5 M LiFSI/DMC), and LSE (1.2 M LiFSI/DMC-BTFE (1:2)) are modeled by using solution of 1 LiFSI:2 DMC, 1 LiFSI:1 DMC, and 1 LiFSI:2 DMC:4 BTFE as three types of adsorption configurations on the most stable Li(100) surface. For the moderately dilute solution, the LiFSI adsorbs with two DMC molecules (FIG. 36F) while only the DMC-LiFSI pair adsorbs the Li(100) surface (FIG. 36D) in the case of the high concentration of LiFSI. As suggested by the previous work (Yamada et al., J Am Chem Soc 2014, 136:5039-5046), the reduction of LiFSI and DMC on the Li anode is examined using the lowest unoccupied molecular orbital (LUMO) energies. FIGS. 43A-43C show projected density of states (PDOS) of each atom in adsorbed LiFSI and DMC molecules for the three configurations. Clearly, in the dilute LiFSI/DMC solution (FIG. 43A), the LUMOs of the conduction bands are located at the DMC molecules. As a result, DMC molecules are reductively decomposed over the Li anode, leading to continuous corrosion of Li metal anode and fast capacity degradation of LMBs upon cycling. In contrast, for the 5.5 M LiFSI concentrated solution (FIG. 43B), the new LUMOs peaks associated with the FSI⁻ anion appear. The lowest energy level of conduction bands of FSI⁻ then is lower than those of DMC, indicating the FSI⁻ anions instead of DMC solvents will be decomposed as the dominant reduction reaction, forming a FSI-derived surface film. Importantly, with dilution with inert BTFE to 1.2 M (FIG. 43C), the Li⁺-DMC-FSI⁻ solvation structures are well maintained, and the LUMOs of the conduction bands are still located on the FSI⁻ anion, rather than the DMC or BTFE molecules. In this regard, FSI⁻ anions are still preferentially reduced to form a robust FSI-derived surface film on Li metal, hence enabling improved stability of LMBs in such a low concentration electrolyte (1.2 M LiFSI/DMC-BTFE).

TABLE 9 DFT calculated adsorption energies and Bader charges of DMC, BTFE, and LiFSI on the Li(100) anode surface. Adsorption energy Bader charge Solvent/salt (kJ · mol⁻¹) (|e|) DMC −54.9 −0.19 BTFE −2.9 −0.06 LiFSI −87.4 −1.97 DMC-LiFSI −148.9 −0.40 2DMC + LIFSI −198.3 −2.14 DMC + DMC-LiFSI −223.2 −2.03

TABLE 10 Mullikan charges of LiFSI salt, DMC and BTFE electrolytes calculated using Gaussian 09 LiFSI DMC BTFE 1 O −0.513511 1 O −0.302932 1 O −0.484765 2 O −0.583827 2 O −0.302935 2 C −0.008499 3 O −0.583829 3 O −0.350118 3 H 0.119682 4 O −0.513513 4 C 0.404178 4 H 0.119680 5 S 1.275462 5 C −0.452945 5 F −0.253411 6 S 1.275462 6 C −0.452947 6 C 0.789574 7 N −0.486613 7 H 0.242245 7 F −0.271213 8 F −0.371232 8 H 0.244359 8 F −0.253404 9 F −0.371232 9 H 0.242246 9 C −0.008475 10 Li 0.872833 10 H 0.244359 10 H 0.119652 11 H 0.242245 11 H 0.119655 12 H 0.242245 12 F −0.253424 13 C 0.789568 14 F −0.271221 15 F −0.253400 LiFSI + DMC LiFSI + BTFE LiFSI + DMC + BTFE 1 O −0.512027 1 O −0.353656 1 O −0.349474 2 O −0.568559 2 C −0.206384 2 C −0.209249 3 O −0.573433 3 C 0.469200 3 C 0.475247 4 O −0.513097 4 C −0.206491 4 C −0.208819 5 S 1.269010 5 C 0.469573 5 C 0.474330 6 S 1.270952 6 H 0.267227 6 H 0.268250 7 N −0.486903 7 H 0.268456 7 H 0.266506 8 F −0.369713 8 H 0.269107 8 H 0.267816 9 F −0.370487 9 H 0.266375 9 H 0.266573 10 Li 0.713270 10 F −0.190602 10 F −0.187169 11 O −0.278551 11 F −0.193998 11 F −0.194059 12 O −0.275832 12 F −0.191335 12 F −0.195904 13 O −0.386995 13 F −0.190062 13 F −0.194621 14 C 0.465780 14 F −0.194829 14 F −0.193822 15 C −0.451501 15 F −0.190852 15 F −0.188438 16 C −0.453985 16 O −0.513004 16 O −0.514689 17 H 0.253230 17 O −0.575146 17 O −0.554408 18 H 0.251398 18 O −0.570364 18 O −0.552391 19 H 0.253073 19 O −0.514274 19 O −0.515485 20 H 0.253083 20 S 1.270093 20 S 1.252134 21 H 0.255599 21 S 1.268717 21 S 1.251197 22 H 0.255689 22 N −0.488169 22 N −0.486900 23 F −0.370392 23 F −0.372415 24 F −0.371383 24 F −0.371326 25 Li 0.772192 25 Li 0.657434 26 O −0.279513 27 O −0.280496 28 O −0.377120 29 C 0.448938 30 C −0.469410 31 C −0.461821 32 H 0.256012 33 H 0.248965 34 H 0.262868 35 H 0.249022 36 H 0.254477 37 H 0.257759 LiFSI: −0.14e LiFSI: −0.09 LiFSI: −0.20 DMC: +0.14e BTFE: +0.09 DMC: +0.11 BTFE: +0.09

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims. 

We claim:
 1. An electrolyte, comprising: an active salt comprising lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), lithium bis(oxalato)borate (LiBOB), sodium bis(oxalato)borate (NaBOB), LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, lithium difluoro oxalato borate anion (LiDFOB), LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof; a solvent comprising dimethoxyethane (DME), dimethyl carbonate (DMC), 1,3-dioxolane (DOL), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl sulfoxide (DMSO), ethyl vinyl sulfone (EVS), tetramethylene sulfone (TMS), ethyl methyl sulfone (EMS), ethylene carbonate (EC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-vinyl-1,3-dioxolan-2-one, dimethyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, triethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, methylene ethylene carbonate (MEC), 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, water, or any combination thereof, wherein the active salt is soluble in the solvent and a molar ratio of the active salt to the solvent is within a range of from 0.7 to 1.5; and a diluent comprising 1,1,2,2-tetrafluoroethyl-2,2,2,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), or any combination thereof, wherein the active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the solvent.
 2. The electrolyte of claim 1, wherein: (i) the active salt has a molar concentration in the electrolyte within a range of from 0.5 M to 2 M; (ii) the active salt has a molar concentration in the solvent of greater than 3 moles of active salt per liter of the solvent; or (iii) both (i) and (ii).
 3. The electrolyte of claim 2, wherein the molar concentration of the active salt in the electrolyte is at least 20% less than a molar concentration of the active salt in the solvent in the absence of the diluent.
 4. The electrolyte of claim 1, wherein: (i) a molar ratio of the active salt to the solvent is within a range of from 0.7-1.2; (ii) a molar ratio of the solvent to the diluent is within a range of from 0.2 to 5; or (iii) both (i) and (ii).
 5. The electrolyte of claim 1, wherein at least 90% of molecules of the solvent are associated with cations of the active salt.
 6. The electrolyte of claim 1, wherein fewer than 10% of molecules of the diluent are associated with cations of the active salt.
 7. The electrolyte of claim 1, wherein: the active salt is (i) LiFSI, LiTFSI, or a combination thereof, or (ii) NaFSI, NaTFSI, or a combination thereof; the solvent is DMC, DME, DOL, EMC, or a combination thereof; and the active salt has a molar concentration in the electrolyte within a range of from 0.75 M to 1.5 M.
 8. The electrolyte of claim 1, wherein the solvent and the diluent are miscible.
 9. The electrolyte of claim 1, wherein the diluent is immiscible with the solvent, the electrolyte further comprising: a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent.
 10. The electrolyte of claim 9, wherein the bridge solvent comprises acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), or any combination thereof.
 11. The electrolyte of claim 9, wherein the solvent comprises water.
 12. The electrolyte of claim 11, wherein the active salt is (i) LiFSI, LiTFSI, LiBOB, LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, LiDFOB, LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof, or (ii) NaFSI, NaTFSI, NaBOB, or a combination thereof.
 13. A battery, comprising: an electrolyte according to claim 1, wherein the active salt has a concentration in the electrolyte within a range of 0.75 to 2 M; an anode; and a cathode, wherein the battery has a coulombic efficiency 95%.
 14. The battery of claim 13, wherein: the anode is lithium metal; the active salt comprises LiFSI, LiTFSI, LiBOB, LiPF₆, LiAsF₆, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiCF₃SO₃, LiClO₄, LiDFOB, LiI, LiBr, LiCl, LiOH, LiNO₃, LiSO₄, or any combination thereof; the solvent comprises DMC, DME, DOL, EVS, TMS, EMS, EC, VC, FEC, 4-methylene-1,3-dioxolan-2-one, dimethyl sulfone, ethyl methyl sulfone, methyl butyrate, ethyl propionate, trimethyl phosphate, gamma-butyrolactone, 4-methylene-1,3-dioxolan-2-one, MEC, 4,5-dimethylene-1,3-dioxolan-2-one, allyl ether, triallyl amine, triallyl cyanurate, triallyl isocyanurate, or any combination thereof; the diluent comprises TTE, BTFE, TFTFE, MOFB, EOFB, or any combination thereof; and the cathode is Li_(1+w)Ni_(x)Mn_(y)Co_(z)O₂ (x+y+z+w=1, 0≤w≤0.25), LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1), LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Mn_(1.5)O₄ spinel, LiMn₂O₄, LiFePO₄, Li_(4−x)M_(x)Ti₅O₁₂ (M=Mg, Al, Ba, Sr, or Ta; 0≤x≤1), MnO₂, V₂O₅, V₆O₁₃, LiV₃O₈, LiM^(C1) _(x)M^(C2) _(1−x)PO₄ (M^(C1) or M^(C2)=Fe, Mn, Ni, Co, Cr, or Ti; 0≤x≤1), Li₃V_(2−x)M¹ _(x)(PO₄)₃ (M¹=Cr, Co, Fe, Mg, Y, Ti, Nb, or Ce; 0≤x≤1), LiVPO₄F, LiM^(C1) _(x)M^(C2) _(1−x)O₂ ((M^(C1) and M^(C2) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1), LiM^(C1) _(x)M^(C2) _(y)M^(C3) _(1−x−y)O₂ ((M^(C1), M^(C2), and M^(C3) independently are Fe, Mn, Ni, Co, Cr, Ti, Mg, or Al; 0≤x≤1; 0≤y≤1), LiMn_(2−y)X_(y)O₄ (X=Cr, Al, or Fe, 0≤y≤1), LiNi_(0.5−y)X_(y)Mn_(1.5)O₄ (X=Fe, Cr, Zn, Al, Mg, Ga, V, or Cu; 0≤y<0.5), xLi₂MnO₃.(1−x)LiM^(C1) _(y)M^(C2) _(z)M^(C3) _(1−y−z)O₂ (M^(C1), M^(C2), and M^(C3) independently are Mn, Ni, Co, Cr, Fe, or mixture thereof; x=0.3-0.5; y≤0.5; z≤0.5), Li₂M²SiO₄ (M²=Mn, Fe, or Co), Li₂M²SO₄ (M²=Mn, Fe, or Co), LiM²SO₄F (M²=Fe, Mn, or Co), Li_(2−x)(Fe_(1−y)Mn_(y))P₂O₇ (0≤y≤), Cr₃O₈, Cr₂O₅, a carbon/sulfur composite, or an air electrode.
 15. The battery of claim 13, wherein: the anode is sodium metal; the active salt comprises NaFSI, NaTFSI, or a combination thereof; the solvent comprises DMC, DME, DOL, or any combination thereof; the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; and the cathode is NaFePO₄, Na₂FePO₄F, Na₂FeP₂O₇, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, NaVPO₄F, NaVPOPOF, Na_(1.5)VOPO₄F_(0.5), NaCo₂O₄, NaFeO₂, Na_(x)MO₂ where 0.4<x≤1, and M is a transition metal or a mixture of transition metals, Na_(2/3)Ni_(1/3)Mn_(2/3)O₂, Na_(2/3)Fe_(1/2)Mn_(1/2)O₂, Na_(2/3)Ni_(1/6)Co_(1/6)Mn_(2/3)O₂, NaNi_(1/3)Fe_(1/3)Mn_(1/3)O₂, NaNi_(1/3)Fe_(1/3)CO_(1/3)O₂, NaNi_(1/2)Mn_(1/2)O₂, a Prussian white analogue cathode, or a Prussian blue analogue cathode.
 16. The battery of claim 13, wherein the solvent and the diluent are immiscible, the electrolyte further comprising a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent.
 17. The battery of claim 16, wherein: the active salt comprises LiFSI, LiTFSI, or a combination thereof; the solvent is H₂O; the bridge solvent comprises AN, DMC, DEC, PC, DMSO, DOL, DME, diglyme, triglyme, tetraglyme, or any combination thereof; the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; the anode is Mo₆S₈, TiO₂, V₂O₅, Li₄Mn₅O₁₂, Li₄Ti₅O₁₂, a C/S composite, or a polyacrylonitrile-sulfur composite; and the cathode is LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1), LiFePO₄, LiMnPO₄, LiFe_(x)Mn_(1−x)PO₄, LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Mn_(1.5)O₄ spinel, or LiMn₂O₄.
 18. The battery of claim 16, wherein: the active salt comprises NaFSI, NaTFSI, or a combination thereof; the solvent is H₂O; the bridge solvent comprises AN, DMC, DEC, PC, DMSO, DOL, DME, diglyme, triglyme, tetraglyme, or any combination thereof; the diluent comprises BTFE, TTE, TFTFE, MOFB, EOFB, or any combination thereof; the anode is NaTi₂(PO₄)₃; TiS₂, CuS, FeS₂, NiCo₂O₄, Cu₂Se, or Li_(0.5)Na_(0.5)Ti₂(PO₄)₃; and the cathode is Na₄Co₃(PO₄)₂P₂O₇, Na₄Co_(2.4)Mn_(0.3)Ni_(0.3)(PO₄)₂P₂O₇, Na₇V₄(P₂O₇)₄PO₄, NaFePO₄, Na₂FePO₄F, Na₂FeP₂O₇, Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, NaVPO₄F, NaVPOPOF, Na_(1.5)VOPO₄F_(0.5), Na₂Fe₂(SO₄)₃, NaFeO₂, and Na_(x)MO₂ where 0.4<x≤1, and M is a transition metal or a mixture of transition metals (e.g., NaCrO₂, NaCoO₂, Na_(x)CoO₂ (0.4≤x≤0.9), Na_(2/3)[Ni_(1/3)Mn_(1/2)Ti_(1/3)]O₂, Na_(2/3)Ni_(1/3)Mn_(2/3)O₂, Na_(2/3)Fe_(1/2)Mn_(1/2)O₂, Na_(2/3)Ni_(1/6)Co_(1/6)Mn_(2/3)O₂, NaNi_(1/3)Fe_(1/3)Mn_(1/3)O₂, NaNi_(1/3)Fe_(1/3)Co_(1/3)O₂, NaNi_(1/2)Mn_(1/2)O₂, a Prussian white analogue cathode, or a Prussian blue analogue cathode.
 19. The electrolyte of claim 1, wherein: the solvent is DME, DOL, or a combination thereof; and the diluent is TTE, BTFE, or a combination thereof.
 20. The electrolyte of claim 7, wherein: the solvent is DME, DOL, or a combination thereof; and the diluent is TTE, BTFE, or a combination thereof.
 21. The electrolyte of claim 20, wherein the solvent is DME. 